added TranslationExtension

Usage:

$app = new Application();
$app->register(new TranslationExtension(), array(
    'translation.class_path' => '...',
));

$app['translator.messages'] = array(
    'en' => array(
        'apple' => 'apple',
    ),
    'fr' => array(
        'apple' => 'pomme',
    ),
);

$app['locale'] = 'fr';

echo $app['translator']->trans('apple');

If you use the Symfony Bridge extension, some tags and filters are also
automatically available in templates:

$templates = array(
    'index' => '{% trans %}apple{% endtrans %}'
);

src/Silex/Extension/TranslationExtension.php

+<?php
+
+/*
+ * This file is part of the Silex framework.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Silex\Extension;
+
+use Silex\Application;
+use Silex\ExtensionInterface;
+
+use Symfony\Component\Translation\Translator;
+use Symfony\Component\Translation\MessageSelector;
+use Symfony\Component\Translation\Loader\ArrayLoader;
+
+class TranslationExtension implements ExtensionInterface
+{
+    public function register(Application $app)
+    {
+        $app['translator'] = $app->share(function() use ($app) {
+            $translator = new Translator(isset($app['locale']) ? $app['locale'] : 'en', $app['translator.message_selector']);
+
+            if (isset($app['locale_fallback'])) {
+                $translator->setFallbackLocale($app['locale_fallback']);
+            }
+
+            $translator->addLoader('array', $app['translator.loader']);
+            foreach ($app['translator.messages'] as $locale => $messages) {
+                $translator->addResource('array', $messages, $locale);
+            }
+
+            return $translator;
+        });
+
+        $app['translator.loader'] = $app->share(function() use ($app) {
+            return new ArrayLoader();
+        });
+
+        $app['translator.message_selector'] = $app->share(function() use ($app) {
+            return new MessageSelector();
+        });
+
+        if (isset($app['translation.class_path'])) {
+            $app['autoloader']->registerNamespace('Symfony\\Component\\Translation', $app['translation.class_path']);
+        }
+    }
+}

src/Silex/Extension/TwigExtension.php

@@ -14,6 +14,7 @@ namespace Silex\Extension;
 use Silex\Application;
 use Silex\ExtensionInterface;
 use Symfony\Bridge\Twig\Extension\RoutingExtension;
+use Symfony\Bridge\Twig\Extension\TransExtension;
 
 class TwigExtension implements ExtensionInterface
 {
@@ -26,8 +27,14 @@ class TwigExtension implements ExtensionInterface
                 $app['twig.configure']($twig);
             }
 
-            if (isset($app['url_generator']) && isset($app['symfony_bridges'])) {
-                $twig->addExtension(new RoutingExtension($app['url_generator']));
+            if (isset($app['symfony_bridges'])) {
+                if (isset($app['url_generator'])) {
+                    $twig->addExtension(new RoutingExtension($app['url_generator']));
+                }
+
+                if (isset($app['translator'])) {
+                    $twig->addExtension(new TransExtension($app['translator']));
+                }
             }
 
             return $twig;