diff --git a/config/initializers/30-redmine.rb b/config/initializers/30-redmine.rb
index 11a248959..1018ca18c 100644
--- a/config/initializers/30-redmine.rb
+++ b/config/initializers/30-redmine.rb
@@ -1,6 +1,5 @@
 I18n.default_locale = 'en'
-# Adds fallback to default locale for untranslated strings
-I18n::Backend::Simple.send(:include, I18n::Backend::Fallbacks)
+I18n.backend = Redmine::I18n::Backend.new
 
 require 'redmine'
 
diff --git a/lib/redmine/i18n.rb b/lib/redmine/i18n.rb
index 12445b450..304175ee5 100644
--- a/lib/redmine/i18n.rb
+++ b/lib/redmine/i18n.rb
@@ -67,7 +67,7 @@ module Redmine
     end
 
     def valid_languages
-      @@valid_languages ||= Dir.glob(File.join(Rails.root, 'config', 'locales', '*.yml')).collect {|f| File.basename(f).split('.').first}.collect(&:to_sym)
+      ::I18n.available_locales
     end
 
     def find_language(lang)
@@ -84,5 +84,74 @@ module Redmine
     def current_language
       ::I18n.locale
     end
+
+    # Custom backend based on I18n::Backend::Simple with the following changes:
+    # * lazy loading of translation files
+    # * available_locales are determined by looking at translation file names
+    class Backend
+      (class << self; self; end).class_eval { public :include }
+
+      module Implementation
+        include ::I18n::Backend::Base
+
+        # Stores translations for the given locale in memory.
+        # This uses a deep merge for the translations hash, so existing
+        # translations will be overwritten by new ones only at the deepest
+        # level of the hash.
+        def store_translations(locale, data, options = {})
+          locale = locale.to_sym
+          translations[locale] ||= {}
+          data = data.deep_symbolize_keys
+          translations[locale].deep_merge!(data)
+        end
+
+        # Get available locales from the translations filenames
+        def available_locales
+          @available_locales ||= ::I18n.load_path.map {|path| File.basename(path, '.*').to_sym}.uniq.sort
+        end
+
+        # Clean up translations
+        def reload!
+          @translations = nil
+          @available_locales = nil
+          super
+        end
+
+        protected
+
+        def init_translations(locale)
+          locale = locale.to_s
+          paths = ::I18n.load_path.select {|path| File.basename(path, '.*') == locale}
+          load_translations(paths)
+          translations[locale] ||= {}
+        end
+
+        def translations
+          @translations ||= {}
+        end
+
+        # Looks up a translation from the translations hash. Returns nil if
+        # eiher key is nil, or locale, scope or key do not exist as a key in the
+        # nested translations hash. Splits keys or scopes containing dots
+        # into multiple keys, i.e. <tt>currency.format</tt> is regarded the same as
+        # <tt>%w(currency format)</tt>.
+        def lookup(locale, key, scope = [], options = {})
+          init_translations(locale) unless translations.key?(locale)
+          keys = ::I18n.normalize_keys(locale, key, scope, options[:separator])
+
+          keys.inject(translations) do |result, _key|
+            _key = _key.to_sym
+            return nil unless result.is_a?(Hash) && result.has_key?(_key)
+            result = result[_key]
+            result = resolve(locale, _key, result, options.merge(:scope => nil)) if result.is_a?(Symbol)
+            result
+          end
+        end
+      end
+
+      include Implementation
+      # Adds fallback to default locale for untranslated strings
+      include ::I18n::Backend::Fallbacks
+    end
   end
 end