Limit lang filter to 2 characters

Ref T805

routes.go

@@ -216,7 +216,7 @@ func InitRoutes(apper Apper, r *mux.Router) *mux.Router {
 func RouteCollections(handler *Handler, r *mux.Router) {
 	r.HandleFunc("/logout", handler.Web(handleLogOutCollection, UserLevelOptional))
 	r.HandleFunc("/page/{page:[0-9]+}", handler.Web(handleViewCollection, UserLevelReader))
-	r.HandleFunc("/lang:{lang}", handler.Web(handleViewCollectionLang, UserLevelOptional))
+	r.HandleFunc("/lang:{lang:[a-z]{2}}", handler.Web(handleViewCollectionLang, UserLevelOptional))
 	r.HandleFunc("/tag:{tag}", handler.Web(handleViewCollectionTag, UserLevelReader))
 	r.HandleFunc("/tag:{tag}/feed/", handler.Web(ViewFeed, UserLevelReader))
 	r.HandleFunc("/sitemap.xml", handler.AllReader(handleViewSitemap))