Provide "I18n::Backend::Fallbacks#on_fallback" hook to allow users to…

lib/i18n/backend/fallbacks.rb

@@ -46,7 +46,10 @@ def translate(locale, key, options = EMPTY_HASH)
           begin
             catch(:exception) do
               result = super(fallback, key, fallback_options)
-              return result unless result.nil?
+              unless result.nil?
+                on_fallback(locale, fallback, key, options) if locale != fallback
+                return result
+              end
             end
           rescue I18n::InvalidLocale
             # we do nothing when the locale is invalid, as this is a fallback anyways.
@@ -80,6 +83,13 @@ def exists?(locale, key, options = EMPTY_HASH)
 
         false
       end
+
+      private
+
+        # Overwrite on_fallback to add specified logic when the fallback succeeds.
+        def on_fallback(_original_locale, _fallback_locale, _key, _optoins)
+          nil
+        end
     end
   end
 end

test/backend/fallbacks_test.rb

@@ -227,3 +227,36 @@ def setup
     assert_equal false, I18n.exists?(:bar, :'de-DE-XX', fallback: false)
   end
 end
+
+class I18nBackendOnFallbackHookTest < I18n::TestCase
+  class Backend < I18n::Backend::Simple
+    include I18n::Backend::Fallbacks
+
+    attr :fallback_collector
+
+    private
+
+      def on_fallback(*args)
+        @fallback_collector ||= []
+        @fallback_collector << args
+      end
+  end
+
+  def setup
+    super
+    I18n.backend = Backend.new
+    I18n.fallbacks = I18n::Locale::Fallbacks.new(de: :en)
+    store_translations(:en, :foo => 'Foo in :en', :bar => 'Bar in :en')
+    store_translations(:de, :bar => 'Bar in :de')
+    store_translations(:"de-DE", :baz => 'Baz in :"de-DE"')
+  end
+
+  test "on_fallback should be called when fallback happens" do
+    assert_equal [:"de-DE", :de, :en], I18n.fallbacks[:"de-DE"]
+    assert_equal 'Baz in :"de-DE"', I18n.t(:baz, locale: :'de-DE')
+    assert_equal 'Bar in :de', I18n.t(:bar, locale: :'de-DE')
+    assert_equal 'Foo in :en', I18n.t(:foo, locale: :'de-DE')
+    assert_equal [:'de-DE', :de, :bar, {}], I18n.backend.fallback_collector[0]
+    assert_equal [:'de-DE', :en, :foo, {}], I18n.backend.fallback_collector[1]
+  end
+end