prevent undefined lang

src/components/DictionaryView.vue

@@ -343,7 +343,7 @@ export default {
         navigate_to_article(self, self.$route.path)
       }
       else {
-        self.lang = self.$route.params.lang
+        self.lang = self.$route.params.lang || self.lang
         self.waiting_for_articles = false
         history.replaceState({article: self.article, search_results: self.search_results, lang: self.lang, error: self.error}, '')
       }