Include I18n::Fallbacks and set the default I18n locale to our default locale.

Thus, when a translation is missing in the current locale, fall back to the default locale. See https://github.com/svenfuchs/i18n/wiki/Fallbacks for more info and configuration settings available to users.

Also, this change adds a "t" helper that proxies to I18n.t, just like Rails, for more concise translating.

middleman-more/features/i18n_preview.feature

@@ -202,4 +202,17 @@ Feature: i18n Preview
     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"'
+    Then I should see '"../stylesheets/site.css"'
+
+  Scenario: Missing translations fall back to the default locale
+    Given a fixture app "i18n-default-app"
+    And a file named "config.rb" with:
+      """
+      activate :i18n, :mount_at_root => :es
+      """
+    Given the Server is running at "i18n-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"

middleman-more/fixtures/i18n-default-app/locales/en.yml

@@ -0,0 +1,4 @@
+---
+en:
+  greetings: "Howdy"
+  hi: "Hello"

middleman-more/fixtures/i18n-default-app/locales/es.yml

@@ -0,0 +1,8 @@
+---
+es:
+  paths:
+    hello: "hola"
+  
+  greetings: "Como Esta?"
+  hi: "Hola"
+  untranslated: "Buenos días"

middleman-more/fixtures/i18n-default-app/source/localizable/index.html.erb

@@ -0,0 +1,5 @@
+Default locale: <%= I18n.default_locale %>
+Current locale: <%= I18n.locale %>
+Fallbacks: <%= I18n.fallbacks[:en] %>
+<%= t(:untranslated) %>
+<%= t(:greetings) %>

middleman-more/lib/middleman-more/core_extensions/i18n.rb

@@ -1,3 +1,6 @@
+require "i18n"
+require "i18n/backend/fallbacks"
+
 module Middleman
   module CoreExtensions
 
@@ -15,6 +18,17 @@ module Middleman
           app.after_configuration do
             Localizer.new(self, options)
           end
+
+          app.helpers do
+            def t(*args)
+              ::I18n.t(*args)
+            end
+          end
+
+          # See https://github.com/svenfuchs/i18n/wiki/Fallbacks
+          unless options[:no_fallbacks]
+            ::I18n::Backend::Simple.send(:include, I18n::Backend::Fallbacks)
+          end
         end
         alias :included :registered
       end
@@ -43,6 +57,12 @@ module Middleman
           @templates_dir = @options[:templates_dir] || "localizable"
           @mount_at_root = @options.has_key?(:mount_at_root) ? @options[:mount_at_root] : langs.first
 
+          ::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
+
           if !@app.build?
             logger.info "== Locales: #{langs.join(", ")} (Default #{@mount_at_root})"
           end
@@ -117,7 +137,7 @@ module Middleman
             langs.map do |lang|
               ::I18n.locale = lang
 
-              localized_page_id = ::I18n.t("paths.#{page_id}", :default => page_id)
+              localized_page_id = ::I18n.t("paths.#{page_id}", :default => page_id, :fallback => [])
               path = resource.path.sub(@templates_dir, "")
 
               # Build lang path