Refactor CoreServiceProvider

A good start I think, but still some work to do. If we go ahead with
https://github.com/flarum/core/issues/132#issuecomment-117507974 (which
I am in favour of), we can extract the entity-related stuff into some
smaller service providers (e.g. discussion repo, an event listener,
permissions, and gambits stuff could all go in
Flarum\Core\Discussions\DiscussionsServiceProvider).

src/Locale/LocaleServiceProvider.php

@@ -0,0 +1,34 @@
+<?php namespace Flarum\Locale;
+
+use Flarum\Support\ServiceProvider;
+use Flarum\Extend;
+
+class LocaleServiceProvider extends ServiceProvider
+{
+    /**
+     * Bootstrap the application events.
+     *
+     * @return void
+     */
+    public function boot()
+    {
+        $this->setupLocale('en');
+    }
+
+    public function setupLocale($locale)
+    {
+        $dir = __DIR__.'/../../locale/'.$locale;
+
+        $this->extend(
+            (new Extend\Locale($locale))
+                ->translations($dir.'/translations.yml')
+                ->config($dir.'/config.php')
+                ->js($dir.'/config.js')
+        );
+    }
+
+    public function register()
+    {
+        $this->app->singleton('flarum.localeManager', 'Flarum\Locale\LocaleManager');
+    }
+}