diff --git a/middleman-core/features/i18n_builder.feature b/middleman-core/features/i18n_v3_builder.feature
similarity index 86%
rename from middleman-core/features/i18n_builder.feature
rename to middleman-core/features/i18n_v3_builder.feature
index 34a802b1..c9b41592 100644
--- a/middleman-core/features/i18n_builder.feature
+++ b/middleman-core/features/i18n_v3_builder.feature
@@ -1,13 +1,13 @@
-Feature: i18n Builder
+Feature: i18n v3 Builder
   In order to preview localized html
 
   Scenario: Running localize with the default config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v3-test-app"
     And a file named "config.rb" with:
       """
-      activate :i18n
+      activate :i18n_v3
       """
-    Given a successfully built app at "i18n-test-app"
+    Given a successfully built app at "i18n-v3-test-app"
     When I cd to "build"
     Then the following files should exist:
       | index.html                                    |
@@ -46,12 +46,12 @@ Feature: i18n Builder
     And the file "password.txt" should contain "hunter2"
 
   Scenario: Running localize with the alt path config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v3-test-app"
     And a file named "config.rb" with:
       """
-      activate :i18n, :path => "/lang_:locale/"
+      activate :i18n_v3, :path => "/lang_:locale/"
       """
-    Given a successfully built app at "i18n-test-app"
+    Given a successfully built app at "i18n-v3-test-app"
     When I cd to "build"
     Then the following files should exist:
       | index.html                                    |
@@ -66,12 +66,12 @@ Feature: i18n Builder
     And the file "lang_es/hola.html" should contain "Hola World"
 
   Scenario: Running localize with the alt root config
-    Given a fixture app "i18n-alt-root-app"
+    Given a fixture app "i18n-v3-alt-root-app"
     And a file named "config.rb" with:
       """
-      activate :i18n, :templates_dir => "lang_data"
+      activate :i18n_v3, :templates_dir => "lang_data"
       """
-    Given a successfully built app at "i18n-alt-root-app"
+    Given a successfully built app at "i18n-v3-alt-root-app"
     When I cd to "build"
     Then the following files should exist:
       | index.html                                    |
@@ -86,12 +86,12 @@ Feature: i18n Builder
     And the file "es/hola.html" should contain "Hola World"
 
   Scenario: Running localize with the lang map config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v3-test-app"
     And a file named "config.rb" with:
       """
-      activate :i18n, :lang_map => { :en => :english, :es => :spanish }
+      activate :i18n_v3, :lang_map => { :en => :english, :es => :spanish }
       """
-    Given a successfully built app at "i18n-test-app"
+    Given a successfully built app at "i18n-v3-test-app"
     When I cd to "build"
     Then the following files should exist:
       | index.html                                    |
@@ -106,12 +106,12 @@ Feature: i18n Builder
     And the file "spanish/hola.html" should contain "Hola World"
 
   Scenario: Running localize with the no mount config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v3-test-app"
     And a file named "config.rb" with:
       """
-      activate :i18n, :mount_at_root => false
+      activate :i18n_v3, :mount_at_root => false
       """
-    Given a successfully built app at "i18n-test-app"
+    Given a successfully built app at "i18n-v3-test-app"
     When I cd to "build"
     Then the following files should exist:
       | en/index.html                                 |
@@ -127,12 +127,12 @@ Feature: i18n Builder
     And the file "es/hola.html" should contain "Hola World"
 
   Scenario: Running localize with the subset config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v3-test-app"
     And a file named "config.rb" with:
       """
-      activate :i18n, :langs => [:en]
+      activate :i18n_v3, :langs => [:en]
       """
-    Given a successfully built app at "i18n-test-app"
+    Given a successfully built app at "i18n-v3-test-app"
     When I cd to "build"
     Then the following files should exist:
       | index.html                                    |
@@ -145,13 +145,13 @@ Feature: i18n Builder
     And the file "hello.html" should contain "Hello World"
 
   Scenario: Running localize with relative_assets
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v3-test-app"
     And a file named "config.rb" with:
       """
-      activate :i18n
+      activate :i18n_v3
       activate :relative_assets
       """
-    Given a successfully built app at "i18n-test-app"
+    Given a successfully built app at "i18n-v3-test-app"
     When I cd to "build"
     Then the following files should exist:
       | index.html                                    |
diff --git a/middleman-core/features/i18n_force_locale.feature b/middleman-core/features/i18n_v3_force_locale.feature
similarity index 80%
rename from middleman-core/features/i18n_force_locale.feature
rename to middleman-core/features/i18n_v3_force_locale.feature
index afe8fc3b..8d57d486 100644
--- a/middleman-core/features/i18n_force_locale.feature
+++ b/middleman-core/features/i18n_v3_force_locale.feature
@@ -1,7 +1,7 @@
-Feature: i18n manually setting locale
+Feature: i18n v3 manually setting locale
 
   Scenario: Setting I18n.locale in a block (see issue #809) or with the :lang option
-    Given the Server is running at "i18n-force-locale"
+    Given the Server is running at "i18n-v3-force-locale"
     When I go to "/en/index.html"
     Then I should see "Hello"
     Then I should see "I18n.locale: en"
diff --git a/middleman-core/features/i18n_v3_preview.feature b/middleman-core/features/i18n_v3_preview.feature
new file mode 100644
index 00000000..47b083fa
--- /dev/null
+++ b/middleman-core/features/i18n_v3_preview.feature
@@ -0,0 +1,252 @@
+Feature: i18n v3 Preview
+  In order to preview localized html
+
+  Scenario: Running localize with the default config
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    When I go to "/"
+    Then I should see "Howdy"
+    When I go to "/hello.html"
+    Then I should see "Hello World"
+    When I go to "/morning.html"
+    Then I should see "Good morning"
+    When I go to "/one.html"
+    Then I should see "Only one"
+    When I go to "/defaults_en/index.html"
+    Then I should see "File Not Found"
+    When I go to "/en/index.html"
+    Then I should see "File Not Found"
+    When I go to "/en/morning.html"
+    Then I should see "File Not Found"
+    When I go to "/defaults_es/index.html"
+    Then I should see "File Not Found"
+    When I go to "/es/index.html"
+    Then I should see "Como Esta?"
+    When I go to "/es/hola.html"
+    Then I should see "Hola World"
+    When I go to "/es/manana.html"
+    Then I should see "Buenos días"
+    When I go to "/es/una.html"
+    Then I should see "Solamente una"
+
+  Scenario: A template changes i18n during preview
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    And the file "locales/en.yml" has the contents
+      """
+      ---
+      en:
+        greetings: "Howdy"
+        hi: "Hello"
+      """
+    When I go to "/"
+    Then I should see "Howdy"
+    When I go to "/hello.html"
+    Then I should see "Hello World"
+    When the file "locales/en.yml" has the contents
+      """
+      ---
+      en:
+        greetings: "How You Doin"
+        hi: "Sup"
+      """
+    When I go to "/"
+    Then I should see "How You Doin"
+    When I go to "/hello.html"
+    Then I should see "Sup World"
+
+  Scenario: Running localize with the alt path config
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3, :path => "/lang_:locale/"
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    When I go to "/"
+    Then I should see "Howdy"
+    When I go to "/hello.html"
+    Then I should see "Hello World"
+    When I go to "/lang_en/index.html"
+    Then I should see "File Not Found"
+    When I go to "/lang_es/index.html"
+    Then I should see "Como Esta?"
+    When I go to "/lang_es/hola.html"
+    Then I should see "Hola World"
+
+
+  Scenario: Running localize with the alt root config
+    Given a fixture app "i18n-v3-alt-root-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3, :templates_dir => "lang_data"
+      """
+    Given the Server is running at "i18n-v3-alt-root-app"
+    When I go to "/"
+    Then I should see "Howdy"
+    When I go to "/hello.html"
+    Then I should see "Hello World"
+    When I go to "/en/index.html"
+    Then I should see "File Not Found"
+    When I go to "/es/index.html"
+    Then I should see "Como Esta?"
+    When I go to "/es/hola.html"
+    Then I should see "Hola World"
+
+  Scenario: Running localize with the lang map config
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3, :lang_map => { :en => :english, :es => :spanish }
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    When I go to "/"
+    Then I should see "Howdy"
+    When I go to "/hello.html"
+    Then I should see "Hello World"
+    When I go to "/english/index.html"
+    Then I should see "File Not Found"
+    When I go to "/spanish/index.html"
+    Then I should see "Como Esta?"
+    When I go to "/spanish/hola.html"
+    Then I should see "Hola World"
+
+  Scenario: Running localize with a non-English mount config
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3, :mount_at_root => :es
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    When I go to "/en/index.html"
+    Then I should see "Howdy"
+    When I go to "/en/hello.html"
+    Then I should see "Hello World"
+    When I go to "/"
+    Then I should see "Como Esta?"
+    When I go to "/hola.html"
+    Then I should see "Hola World"
+    When I go to "/manana.html"
+    Then I should see "Buenos días"
+    When I go to "/hello.html"
+    Then I should see "File Not Found"
+    When I go to "/en/morning.html"
+    Then I should see "Good morning"
+    When I go to "/es/manana.html"
+    Then I should see "File Not Found"
+    When I go to "/es/index.html"
+    Then I should see "File Not Found"
+    When I go to "/es/hola.html"
+    Then I should see "File Not Found"
+
+  Scenario: Running localize with a non-English lang subset
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3, :langs => :es
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    When I go to "/en/index.html"
+    Then I should see "File Not Found"
+    When I go to "/en/hello.html"
+    Then I should see "File Not Found"
+    When I go to "/"
+    Then I should see "Como Esta?"
+    When I go to "/hola.html"
+    Then I should see "Hola World"
+    When I go to "/hello.html"
+    Then I should see "File Not Found"
+    When I go to "/es/index.html"
+    Then I should see "File Not Found"
+    When I go to "/es/hola.html"
+    Then I should see "File Not Found"
+
+
+  Scenario: Running localize with the no mount config
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3, :mount_at_root => false
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    When I go to "/en/index.html"
+    Then I should see "Howdy"
+    When I go to "/en/hello.html"
+    Then I should see "Hello World"
+    When I go to "/"
+    Then I should see "File Not Found"
+    When I go to "/hello.html"
+    Then I should see "File Not Found"
+    When I go to "/es/index.html"
+    Then I should see "Como Esta?"
+    When I go to "/es/hola.html"
+    Then I should see "Hola World"
+
+  Scenario: Running localize with the subset config
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3, :langs => [:en]
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    When I go to "/"
+    Then I should see "Howdy"
+    When I go to "/hello.html"
+    Then I should see "Hello World"
+    When I go to "/en/index.html"
+    Then I should see "File Not Found"
+    When I go to "/es/index.html"
+    Then I should see "File Not Found"
+    When I go to "/es/hola.html"
+    Then I should see "File Not Found"
+
+  Scenario: Running localize with relative_assets
+    Given a fixture app "i18n-v3-test-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3
+      activate :relative_assets
+      """
+    Given the Server is running at "i18n-v3-test-app"
+    When I go to "/"
+    Then I should see '"stylesheets/site.css"'
+    When I go to "/hello.html"
+    Then I should see '"stylesheets/site.css"'
+    When I go to "/es/index.html"
+    Then I should see '"../stylesheets/site.css"'
+    When I go to "/es/hola.html"
+    Then I should see '"../stylesheets/site.css"'
+
+  Scenario: Missing translations fall back to the default locale
+    Given a fixture app "i18n-v3-default-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3, :mount_at_root => :es
+      """
+    Given the Server is running at "i18n-v3-default-app"
+    When I go to "/en/"
+    Then I should see "Default locale: es"
+    Then I should see "Current locale: en"
+    Then I should see "Buenos días"
+    Then I should see "Howdy"
+
+  Scenario: Nested i18n yaml
+    Given a fixture app "i18n-v3-nested-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n_v3
+      """
+    Given the Server is running at "i18n-v3-nested-app"
+    When I go to "/"
+    Then I should see "Howdy"
+    Then I should see "More"
+    When I go to "/es/"
+    Then I should see "Como Esta?"
+    Then I should see "Mucho"
\ No newline at end of file
diff --git a/middleman-core/features/i18n_preview.feature b/middleman-core/features/i18n_v4_preview.feature
similarity index 84%
rename from middleman-core/features/i18n_preview.feature
rename to middleman-core/features/i18n_v4_preview.feature
index 93c60dc1..df9bd0b0 100644
--- a/middleman-core/features/i18n_preview.feature
+++ b/middleman-core/features/i18n_v4_preview.feature
@@ -1,13 +1,14 @@
-Feature: i18n Preview
+@wip
+Feature: i18n v4 Preview
   In order to preview localized html
 
   Scenario: Running localize with the default config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     When I go to "/"
     Then I should see "Howdy"
     When I go to "/hello.html"
@@ -34,12 +35,12 @@ Feature: i18n Preview
     Then I should see "Solamente una"
 
   Scenario: A template changes i18n during preview
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     And the file "locales/en.yml" has the contents
       """
       ---
@@ -64,12 +65,12 @@ Feature: i18n Preview
     Then I should see "Sup World"
 
   Scenario: Running localize with the alt path config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n, :path => "/lang_:locale/"
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     When I go to "/"
     Then I should see "Howdy"
     When I go to "/hello.html"
@@ -83,12 +84,12 @@ Feature: i18n Preview
 
 
   Scenario: Running localize with the alt root config
-    Given a fixture app "i18n-alt-root-app"
+    Given a fixture app "i18n-v4-alt-root-app"
     And a file named "config.rb" with:
       """
       activate :i18n, :templates_dir => "lang_data"
       """
-    Given the Server is running at "i18n-alt-root-app"
+    Given the Server is running at "i18n-v4-alt-root-app"
     When I go to "/"
     Then I should see "Howdy"
     When I go to "/hello.html"
@@ -101,12 +102,12 @@ Feature: i18n Preview
     Then I should see "Hola World"
 
   Scenario: Running localize with the lang map config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n, :lang_map => { :en => :english, :es => :spanish }
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     When I go to "/"
     Then I should see "Howdy"
     When I go to "/hello.html"
@@ -119,12 +120,12 @@ Feature: i18n Preview
     Then I should see "Hola World"
 
   Scenario: Running localize with a non-English mount config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n, :mount_at_root => :es
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     When I go to "/en/index.html"
     Then I should see "Howdy"
     When I go to "/en/hello.html"
@@ -147,12 +148,12 @@ Feature: i18n Preview
     Then I should see "File Not Found"
 
   Scenario: Running localize with a non-English lang subset
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n, :langs => :es
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     When I go to "/en/index.html"
     Then I should see "File Not Found"
     When I go to "/en/hello.html"
@@ -170,12 +171,12 @@ Feature: i18n Preview
 
 
   Scenario: Running localize with the no mount config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n, :mount_at_root => false
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     When I go to "/en/index.html"
     Then I should see "Howdy"
     When I go to "/en/hello.html"
@@ -190,12 +191,12 @@ Feature: i18n Preview
     Then I should see "Hola World"
 
   Scenario: Running localize with the subset config
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n, :langs => [:en]
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     When I go to "/"
     Then I should see "Howdy"
     When I go to "/hello.html"
@@ -208,13 +209,13 @@ Feature: i18n Preview
     Then I should see "File Not Found"
 
   Scenario: Running localize with relative_assets
-    Given a fixture app "i18n-test-app"
+    Given a fixture app "i18n-v4-test-app"
     And a file named "config.rb" with:
       """
       activate :i18n
       activate :relative_assets
       """
-    Given the Server is running at "i18n-test-app"
+    Given the Server is running at "i18n-v4-test-app"
     When I go to "/"
     Then I should see '"stylesheets/site.css"'
     When I go to "/hello.html"
@@ -225,12 +226,12 @@ Feature: i18n Preview
     Then I should see '"../stylesheets/site.css"'
 
   Scenario: Missing translations fall back to the default locale
-    Given a fixture app "i18n-default-app"
+    Given a fixture app "i18n-v4-default-app"
     And a file named "config.rb" with:
       """
       activate :i18n, :mount_at_root => :es
       """
-    Given the Server is running at "i18n-default-app"
+    Given the Server is running at "i18n-v4-default-app"
     When I go to "/en/"
     Then I should see "Default locale: es"
     Then I should see "Current locale: en"
@@ -238,12 +239,12 @@ Feature: i18n Preview
     Then I should see "Howdy"
 
   Scenario: Nested i18n yaml
-    Given a fixture app "i18n-nested-app"
+    Given a fixture app "i18n-v4-nested-app"
     And a file named "config.rb" with:
       """
       activate :i18n
       """
-    Given the Server is running at "i18n-nested-app"
+    Given the Server is running at "i18n-v4-nested-app"
     When I go to "/"
     Then I should see "Howdy"
     Then I should see "More"
diff --git a/middleman-core/fixtures/i18n-alt-root-app/locales/en.yml b/middleman-core/fixtures/i18n-v3-alt-root-app/locales/en.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-alt-root-app/locales/en.yml
rename to middleman-core/fixtures/i18n-v3-alt-root-app/locales/en.yml
diff --git a/middleman-core/fixtures/i18n-alt-root-app/locales/es.yml b/middleman-core/fixtures/i18n-v3-alt-root-app/locales/es.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-alt-root-app/locales/es.yml
rename to middleman-core/fixtures/i18n-v3-alt-root-app/locales/es.yml
diff --git a/middleman-core/fixtures/i18n-alt-root-app/source/lang_data/hello.html.erb b/middleman-core/fixtures/i18n-v3-alt-root-app/source/lang_data/hello.html.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-alt-root-app/source/lang_data/hello.html.erb
rename to middleman-core/fixtures/i18n-v3-alt-root-app/source/lang_data/hello.html.erb
diff --git a/middleman-core/fixtures/i18n-alt-root-app/source/lang_data/index.html.erb b/middleman-core/fixtures/i18n-v3-alt-root-app/source/lang_data/index.html.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-alt-root-app/source/lang_data/index.html.erb
rename to middleman-core/fixtures/i18n-v3-alt-root-app/source/lang_data/index.html.erb
diff --git a/middleman-core/fixtures/i18n-alt-root-app/source/layout.erb b/middleman-core/fixtures/i18n-v3-alt-root-app/source/layout.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-alt-root-app/source/layout.erb
rename to middleman-core/fixtures/i18n-v3-alt-root-app/source/layout.erb
diff --git a/middleman-core/fixtures/i18n-default-app/locales/en.yml b/middleman-core/fixtures/i18n-v3-default-app/locales/en.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-default-app/locales/en.yml
rename to middleman-core/fixtures/i18n-v3-default-app/locales/en.yml
diff --git a/middleman-core/fixtures/i18n-default-app/locales/es.yml b/middleman-core/fixtures/i18n-v3-default-app/locales/es.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-default-app/locales/es.yml
rename to middleman-core/fixtures/i18n-v3-default-app/locales/es.yml
diff --git a/middleman-core/fixtures/i18n-default-app/source/localizable/index.html.erb b/middleman-core/fixtures/i18n-v3-default-app/source/localizable/index.html.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-default-app/source/localizable/index.html.erb
rename to middleman-core/fixtures/i18n-v3-default-app/source/localizable/index.html.erb
diff --git a/middleman-core/fixtures/i18n-force-locale/config.rb b/middleman-core/fixtures/i18n-v3-force-locale/config.rb
similarity index 94%
rename from middleman-core/fixtures/i18n-force-locale/config.rb
rename to middleman-core/fixtures/i18n-v3-force-locale/config.rb
index 4058360e..e0730412 100644
--- a/middleman-core/fixtures/i18n-force-locale/config.rb
+++ b/middleman-core/fixtures/i18n-v3-force-locale/config.rb
@@ -4,7 +4,7 @@ end
 
 proxy "/fr/index.html", "index.html", :lang => :fr
 
-activate :i18n
+activate :i18n_v3
 
 # This is what breaks i18n, just because it adds a resource list manipulator that 
 # forces a rebuild of the resource list.
diff --git a/middleman-core/fixtures/i18n-force-locale/locales/en.yml b/middleman-core/fixtures/i18n-v3-force-locale/locales/en.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-force-locale/locales/en.yml
rename to middleman-core/fixtures/i18n-v3-force-locale/locales/en.yml
diff --git a/middleman-core/fixtures/i18n-force-locale/locales/es.yml b/middleman-core/fixtures/i18n-v3-force-locale/locales/es.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-force-locale/locales/es.yml
rename to middleman-core/fixtures/i18n-v3-force-locale/locales/es.yml
diff --git a/middleman-core/fixtures/i18n-force-locale/locales/fr.yml b/middleman-core/fixtures/i18n-v3-force-locale/locales/fr.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-force-locale/locales/fr.yml
rename to middleman-core/fixtures/i18n-v3-force-locale/locales/fr.yml
diff --git a/middleman-core/fixtures/i18n-force-locale/source/index.html.haml b/middleman-core/fixtures/i18n-v3-force-locale/source/index.html.haml
similarity index 100%
rename from middleman-core/fixtures/i18n-force-locale/source/index.html.haml
rename to middleman-core/fixtures/i18n-v3-force-locale/source/index.html.haml
diff --git a/middleman-core/fixtures/i18n-nested-app/locales/en.yml b/middleman-core/fixtures/i18n-v3-nested-app/locales/en.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-nested-app/locales/en.yml
rename to middleman-core/fixtures/i18n-v3-nested-app/locales/en.yml
diff --git a/middleman-core/fixtures/i18n-nested-app/locales/en/more.yml b/middleman-core/fixtures/i18n-v3-nested-app/locales/en/more.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-nested-app/locales/en/more.yml
rename to middleman-core/fixtures/i18n-v3-nested-app/locales/en/more.yml
diff --git a/middleman-core/fixtures/i18n-nested-app/locales/es.yml b/middleman-core/fixtures/i18n-v3-nested-app/locales/es.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-nested-app/locales/es.yml
rename to middleman-core/fixtures/i18n-v3-nested-app/locales/es.yml
diff --git a/middleman-core/fixtures/i18n-nested-app/locales/es/mucho.yml b/middleman-core/fixtures/i18n-v3-nested-app/locales/es/mucho.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-nested-app/locales/es/mucho.yml
rename to middleman-core/fixtures/i18n-v3-nested-app/locales/es/mucho.yml
diff --git a/middleman-core/fixtures/i18n-nested-app/source/localizable/index.html.erb b/middleman-core/fixtures/i18n-v3-nested-app/source/localizable/index.html.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-nested-app/source/localizable/index.html.erb
rename to middleman-core/fixtures/i18n-v3-nested-app/source/localizable/index.html.erb
diff --git a/middleman-core/fixtures/i18n-test-app/data/defaults_en.yml b/middleman-core/fixtures/i18n-v3-test-app/data/defaults_en.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/data/defaults_en.yml
rename to middleman-core/fixtures/i18n-v3-test-app/data/defaults_en.yml
diff --git a/middleman-core/fixtures/i18n-test-app/data/defaults_es.yml b/middleman-core/fixtures/i18n-v3-test-app/data/defaults_es.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/data/defaults_es.yml
rename to middleman-core/fixtures/i18n-v3-test-app/data/defaults_es.yml
diff --git a/middleman-core/fixtures/i18n-test-app/data/en_defaults.yml b/middleman-core/fixtures/i18n-v3-test-app/data/en_defaults.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/data/en_defaults.yml
rename to middleman-core/fixtures/i18n-v3-test-app/data/en_defaults.yml
diff --git a/middleman-core/fixtures/i18n-test-app/locales/en.yml b/middleman-core/fixtures/i18n-v3-test-app/locales/en.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/locales/en.yml
rename to middleman-core/fixtures/i18n-v3-test-app/locales/en.yml
diff --git a/middleman-core/fixtures/i18n-test-app/locales/es.yml b/middleman-core/fixtures/i18n-v3-test-app/locales/es.yml
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/locales/es.yml
rename to middleman-core/fixtures/i18n-v3-test-app/locales/es.yml
diff --git a/middleman-core/fixtures/i18n-test-app/source/CNAME b/middleman-core/fixtures/i18n-v3-test-app/source/CNAME
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/CNAME
rename to middleman-core/fixtures/i18n-v3-test-app/source/CNAME
diff --git a/middleman-core/fixtures/i18n-test-app/source/layouts/layout.erb b/middleman-core/fixtures/i18n-v3-test-app/source/layouts/layout.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/layouts/layout.erb
rename to middleman-core/fixtures/i18n-v3-test-app/source/layouts/layout.erb
diff --git a/middleman-core/fixtures/i18n-test-app/source/localizable/hello.html.erb b/middleman-core/fixtures/i18n-v3-test-app/source/localizable/hello.html.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/localizable/hello.html.erb
rename to middleman-core/fixtures/i18n-v3-test-app/source/localizable/hello.html.erb
diff --git a/middleman-core/fixtures/i18n-test-app/source/localizable/index.html.erb b/middleman-core/fixtures/i18n-v3-test-app/source/localizable/index.html.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/localizable/index.html.erb
rename to middleman-core/fixtures/i18n-v3-test-app/source/localizable/index.html.erb
diff --git a/middleman-core/fixtures/i18n-test-app/source/localizable/morning.en.html.erb b/middleman-core/fixtures/i18n-v3-test-app/source/localizable/morning.en.html.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/localizable/morning.en.html.erb
rename to middleman-core/fixtures/i18n-v3-test-app/source/localizable/morning.en.html.erb
diff --git a/middleman-core/fixtures/i18n-test-app/source/localizable/morning.es.html.erb b/middleman-core/fixtures/i18n-v3-test-app/source/localizable/morning.es.html.erb
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/localizable/morning.es.html.erb
rename to middleman-core/fixtures/i18n-v3-test-app/source/localizable/morning.es.html.erb
diff --git a/middleman-core/fixtures/i18n-test-app/source/localizable/one.en.html.md b/middleman-core/fixtures/i18n-v3-test-app/source/localizable/one.en.html.md
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/localizable/one.en.html.md
rename to middleman-core/fixtures/i18n-v3-test-app/source/localizable/one.en.html.md
diff --git a/middleman-core/fixtures/i18n-test-app/source/localizable/one.es.html.md b/middleman-core/fixtures/i18n-v3-test-app/source/localizable/one.es.html.md
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/localizable/one.es.html.md
rename to middleman-core/fixtures/i18n-v3-test-app/source/localizable/one.es.html.md
diff --git a/middleman-core/fixtures/i18n-test-app/source/password.txt b/middleman-core/fixtures/i18n-v3-test-app/source/password.txt
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/password.txt
rename to middleman-core/fixtures/i18n-v3-test-app/source/password.txt
diff --git a/middleman-core/fixtures/i18n-test-app/source/stylesheets/site.css b/middleman-core/fixtures/i18n-v3-test-app/source/stylesheets/site.css
similarity index 100%
rename from middleman-core/fixtures/i18n-test-app/source/stylesheets/site.css
rename to middleman-core/fixtures/i18n-v3-test-app/source/stylesheets/site.css
diff --git a/middleman-core/fixtures/i18n-v4-test-app/data/defaults_en.yml b/middleman-core/fixtures/i18n-v4-test-app/data/defaults_en.yml
new file mode 100644
index 00000000..e69de29b
diff --git a/middleman-core/fixtures/i18n-v4-test-app/data/defaults_es.yml b/middleman-core/fixtures/i18n-v4-test-app/data/defaults_es.yml
new file mode 100644
index 00000000..e69de29b
diff --git a/middleman-core/fixtures/i18n-v4-test-app/data/en_defaults.yml b/middleman-core/fixtures/i18n-v4-test-app/data/en_defaults.yml
new file mode 100644
index 00000000..e69de29b
diff --git a/middleman-core/fixtures/i18n-v4-test-app/locales/en.yml b/middleman-core/fixtures/i18n-v4-test-app/locales/en.yml
new file mode 100644
index 00000000..91519342
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/locales/en.yml
@@ -0,0 +1,4 @@
+---
+en:
+  greetings: "Howdy"
+  hi: "Hello"
\ No newline at end of file
diff --git a/middleman-core/fixtures/i18n-v4-test-app/locales/es.yml b/middleman-core/fixtures/i18n-v4-test-app/locales/es.yml
new file mode 100644
index 00000000..a3321546
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/locales/es.yml
@@ -0,0 +1,9 @@
+---
+es:
+  paths:
+    hello: "hola"
+    morning: "manana"
+    one: "una"
+  
+  greetings: "Como Esta?"
+  hi: "Hola"
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/CNAME b/middleman-core/fixtures/i18n-v4-test-app/source/CNAME
new file mode 100644
index 00000000..0aca0c56
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/CNAME
@@ -0,0 +1 @@
+test.github.com
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/hello.html.erb b/middleman-core/fixtures/i18n-v4-test-app/source/hello.html.erb
new file mode 100644
index 00000000..a226480d
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/hello.html.erb
@@ -0,0 +1 @@
+<%= I18n.t(:hi) %> World
\ No newline at end of file
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/index.html.erb b/middleman-core/fixtures/i18n-v4-test-app/source/index.html.erb
new file mode 100644
index 00000000..c39d59f5
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/index.html.erb
@@ -0,0 +1 @@
+<%= I18n.t(:greetings) %>
\ No newline at end of file
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/layouts/layout.erb b/middleman-core/fixtures/i18n-v4-test-app/source/layouts/layout.erb
new file mode 100644
index 00000000..cb465e99
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/layouts/layout.erb
@@ -0,0 +1,8 @@
+<html>
+  <head>
+    <%= stylesheet_link_tag :site %>
+  </head>
+  <body>
+    <%= yield %>
+  </body>
+</html>
\ No newline at end of file
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/morning.en.html.erb b/middleman-core/fixtures/i18n-v4-test-app/source/morning.en.html.erb
new file mode 100644
index 00000000..9fc8e015
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/morning.en.html.erb
@@ -0,0 +1 @@
+Good morning
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/morning.es.html.erb b/middleman-core/fixtures/i18n-v4-test-app/source/morning.es.html.erb
new file mode 100644
index 00000000..30d65605
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/morning.es.html.erb
@@ -0,0 +1 @@
+Buenos días
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/one.en.html.md b/middleman-core/fixtures/i18n-v4-test-app/source/one.en.html.md
new file mode 100644
index 00000000..2c4dcf4f
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/one.en.html.md
@@ -0,0 +1 @@
+Only one
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/one.es.html.md b/middleman-core/fixtures/i18n-v4-test-app/source/one.es.html.md
new file mode 100644
index 00000000..8d31bd0f
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/one.es.html.md
@@ -0,0 +1 @@
+Solamente una
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/password.txt b/middleman-core/fixtures/i18n-v4-test-app/source/password.txt
new file mode 100644
index 00000000..9f592eb7
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/password.txt
@@ -0,0 +1 @@
+hunter2
diff --git a/middleman-core/fixtures/i18n-v4-test-app/source/stylesheets/site.css b/middleman-core/fixtures/i18n-v4-test-app/source/stylesheets/site.css
new file mode 100644
index 00000000..ee28067a
--- /dev/null
+++ b/middleman-core/fixtures/i18n-v4-test-app/source/stylesheets/site.css
@@ -0,0 +1,3 @@
+body {
+  hello: world;
+}
\ No newline at end of file
diff --git a/middleman-core/lib/middleman-core/core_extensions.rb b/middleman-core/lib/middleman-core/core_extensions.rb
index 340ca2fd..705b76f7 100644
--- a/middleman-core/lib/middleman-core/core_extensions.rb
+++ b/middleman-core/lib/middleman-core/core_extensions.rb
@@ -45,8 +45,13 @@ end
 # Setup Optional Extensions
 ###
 
+Middleman::Extensions.register :i18n_v3 do
+  require 'middleman-core/core_extensions/i18n_v3'
+  Middleman::CoreExtensions::InternationalizationV3
+end
+
 Middleman::Extensions.register :i18n do
-  require 'middleman-core/core_extensions/i18n'
+  require 'middleman-core/core_extensions/i18n_v4'
   Middleman::CoreExtensions::Internationalization
 end
 
diff --git a/middleman-core/lib/middleman-core/core_extensions/i18n.rb b/middleman-core/lib/middleman-core/core_extensions/i18n_v3.rb
similarity index 98%
rename from middleman-core/lib/middleman-core/core_extensions/i18n.rb
rename to middleman-core/lib/middleman-core/core_extensions/i18n_v3.rb
index f500d953..7d1cfaaf 100644
--- a/middleman-core/lib/middleman-core/core_extensions/i18n.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/i18n_v3.rb
@@ -1,4 +1,4 @@
-class Middleman::CoreExtensions::Internationalization < ::Middleman::Extension
+class Middleman::CoreExtensions::InternationalizationV3 < ::Middleman::Extension
   option :no_fallbacks, false, 'Disable I18n fallbacks'
   option :langs, nil, 'List of langs, will autodiscover by default'
   option :lang_map, {}, 'Language shortname map'
diff --git a/middleman-core/lib/middleman-core/core_extensions/i18n_v4.rb b/middleman-core/lib/middleman-core/core_extensions/i18n_v4.rb
new file mode 100644
index 00000000..6abb1e7e
--- /dev/null
+++ b/middleman-core/lib/middleman-core/core_extensions/i18n_v4.rb
@@ -0,0 +1,194 @@
+class Middleman::CoreExtensions::Internationalization < ::Middleman::Extension
+  option :no_fallbacks, false, 'Disable I18n fallbacks'
+  option :langs, nil, 'List of langs, will autodiscover by default'
+  option :lang_map, {}, 'Language shortname map'
+  option :path, '/:locale/', 'URL prefix path'
+  option :files_glob, ["**/*.html"], 'Files to automatically localize'
+  option :mount_at_root, nil, 'Mount a specific language at the root of the site'
+  option :data, 'locales', 'The directory holding your locale configurations'
+
+  def initialize(app, options_hash={}, &block)
+    super
+
+    # See https://github.com/svenfuchs/i18n/wiki/Fallbacks
+    unless options[:no_fallbacks]
+      require 'i18n/backend/fallbacks'
+      ::I18n::Backend::Simple.send(:include, ::I18n::Backend::Fallbacks)
+    end
+
+    app.send :include, LocaleHelpers
+  end
+
+  def after_configuration
+    # app.files.reload_path(app.config[:locals_dir] || options[:data])
+
+    # @locales_glob = File.join(app.config[:locals_dir] || options[:data], '**', '*.{rb,yml,yaml}')
+    # @locales_regex = convert_glob_to_regex(@locales_glob)
+
+    # @maps = {}
+    # @mount_at_root = options[:mount_at_root].nil? ? langs.first : options[:mount_at_root]
+
+    # configure_i18n
+
+    # if !app.build?
+    #   logger.info "== Locales: #{langs.join(", ")} (Default #{@mount_at_root})"
+    # end
+
+    # app.sitemap.provides_metadata_for_path(&method(:metadata_for_path))
+    # app.files.changed(&method(:on_file_changed))
+    # app.files.deleted(&method(:on_file_changed))
+  end
+
+  helpers do
+    def t(*args)
+      ::I18n.t(*args)
+    end
+  end
+
+  delegate :logger, :to => :app
+
+  def langs
+    @_langs ||= get_known_languages
+  end
+
+  # Update the main sitemap resource list
+  # @return [void]
+  # def manipulate_resource_list(resources)
+  #   @_localization_data = {}
+
+  #   new_resources = []
+
+  #   resources.each do |resource|
+  #     # If it uses file extension localization
+  #     if !parse_locale_extension(resource.path).nil?
+  #       result = parse_locale_extension(resource.path)
+  #       lang, path, page_id = result
+  #       new_resources << build_resource(path, resource.path, page_id, lang)
+  #     # If it's a "localizable template"
+  #     elsif File.fnmatch?(File.join(options[:templates_dir], '**'), resource.path)
+  #       page_id = File.basename(resource.path, File.extname(resource.path))
+  #       langs.each do |lang|
+  #         # Remove folder name
+  #         path = resource.path.sub(options[:templates_dir], '')
+  #         new_resources << build_resource(path, resource.path, page_id, lang)
+  #       end
+  #     end
+  #   end
+
+  #   resources + new_resources
+  # end
+
+  private
+
+  def on_file_changed(file)
+    if @locales_regex =~ file
+      @_langs = nil # Clear langs cache
+      ::I18n.reload!
+    end
+  end
+
+  def convert_glob_to_regex(glob)
+    # File.fnmatch doesn't support brackets: {rb,yml,yaml}
+    regex = @locales_glob.sub(/\./, '\.').sub(File.join('**', '*'), '.*').sub(/\//, '\/').sub('{rb,yml,yaml}', '(rb|ya?ml)')
+    %r{^#{regex}}
+  end
+
+  def configure_i18n
+    ::I18n.load_path += Dir[File.join(app.root, @locales_glob)]
+    ::I18n.reload!
+
+    ::I18n.default_locale = @mount_at_root
+    # Reset fallbacks to fall back to our new default
+    if ::I18n.respond_to? :fallbacks
+      ::I18n.fallbacks = ::I18n::Locale::Fallbacks.new
+    end
+  end
+
+  def metadata_for_path(url)
+    if d = get_localization_data(url)
+      lang, page_id = d
+    else
+      # Default to the @mount_at_root lang
+      page_id = nil
+      lang = @mount_at_root
+    end
+
+    {
+      :locals => {
+        :lang => lang,
+        :page_id => page_id
+      },
+      :options => { :lang => lang }
+    }
+  end
+
+  def get_known_languages
+    if options[:langs]
+      Array(options[:langs]).map(&:to_sym)
+    else
+      known_langs = app.files.known_paths.select do |p|
+        p.to_s.match(@locales_regex) && (p.to_s.split(File::SEPARATOR).length === 2)
+      end.map { |p|
+        File.basename(p.to_s).sub(/\.ya?ml$/, '').sub(/\.rb$/, '')
+      }.sort.map(&:to_sym)
+    end
+  end
+
+  def get_localization_data(path)
+    @_localization_data ||= {}
+    @_localization_data[path]
+  end
+
+  # Parse locale extension filename
+  # @return [lang, path, basename]
+  # will return +nil+ if no locale extension
+  def parse_locale_extension(path)
+    path_bits = path.split('.')
+    return nil if path_bits.size < 3
+
+    lang = path_bits.delete_at(-2).to_sym
+    return nil unless langs.include?(lang)
+
+    path = path_bits.join('.')
+    basename = File.basename(path_bits[0..-2].join('.'))
+    [lang, path, basename]
+  end
+
+  def build_resource(path, source_path, page_id, lang)
+    old_locale = ::I18n.locale
+    ::I18n.locale = lang
+    localized_page_id = ::I18n.t("paths.#{page_id}", :default => page_id, :fallback => [])
+
+    prefix = if (options[:mount_at_root] == lang) || (options[:mount_at_root] == nil && langs[0] == lang)
+      '/'
+    else
+      replacement = options[:lang_map].fetch(lang, lang)
+      options[:path].sub(':locale', replacement.to_s)
+    end
+
+    # path needs to be changed if file has a localizable extension. (options[mount_at_root] == lang)
+    path = ::Middleman::Util.normalize_path(
+      File.join(prefix, path.sub(page_id, localized_page_id))
+    )
+
+    path.gsub!(options[:templates_dir]+'/', '')
+
+    @_localization_data[path] = [lang, path, localized_page_id]
+
+    p = ::Middleman::Sitemap::Resource.new(app.sitemap, path)
+    p.proxy_to(source_path)
+
+    ::I18n.locale = old_locale
+    p
+  end
+
+  module LocaleHelpers
+    # Access the list of languages supported by this Middleman application
+    # @return [Array<Symbol>]
+    def langs
+      extensions[:i18n].langs
+    end
+  end
+end
+
+Middleman::CoreExtensions::Internationalization.register(:i18n)