Implemented determine_language_given_path for Flash formatter.

lib/twine/formatters/flash.rb

@@ -20,7 +20,8 @@ module Twine
       end
 
       def determine_language_given_path(path)
-        return
+        # match two-letter language code, optionally followed by a two letter region code
+        path.split(File::SEPARATOR).reverse.find { |segment| segment =~ /^([a-z]{2}(-[a-z]{2})?)$/i }
       end
 
       def set_translation_for_key(key, lang, value)

test/test_formatters.rb

@@ -352,4 +352,13 @@ class TestFlashFormatter < FormatterTest
   def test_format_value_converts_placeholders
     assert_equal "value {0}", @formatter.format_value('value %d')
   end
+
+  def test_deducts_language_from_resource_folder
+    language = %w(en de fr).sample
+    assert_equal language, @formatter.determine_language_given_path("locale/#{language}")
+  end
+
+  def test_deducts_language_and_region_from_resource_folder
+    assert_equal 'de-AT', @formatter.determine_language_given_path("locale/de-AT")
+  end
 end