Fix broken language switch when moving in browser history (#59)

packages/app-shell/src/app-shell.js

@@ -44,6 +44,7 @@ export class AppShell extends ScopedElementsMixin(DBPLitElement) {
     constructor() {
         super();
         this.lang = i18n.language;
+        this.languages = [];
         this.activeView = '';
         this.entryPointUrl = '';
         this.subtitle = '';
@@ -270,8 +271,12 @@ export class AppShell extends ScopedElementsMixin(DBPLitElement) {
      * @param {string} lang
      */
     updateLangIfChanged(lang) {
+        // if the language is switched to English then i18n.languages is ['en'], instead of ['en', 'de']
+        // so we are now only adding new ones to this.languages
+        this.languages = Array.from(new Set(this.languages.concat(i18n.languages)));
+
         // in case the language is unknown, fall back to the default
-        if (!i18n.languages.includes(lang)) {
+        if (!this.languages.includes(lang)) {
             lang = this.lang;
         }
         if (this.lang !== lang) {