Unknown languages now lead to English UI. Updated documentation

on how internationalization works.

README.md

@@ -19,3 +19,10 @@ To activate playlist functionality, set one of the following attributes:
  * `list-local`: A string containing a playlist definition.
 
 An arbitrary number of players can be placed on the same page.
+
+
+## Internationalization
+
+The player defaults to English as its UI language. If the containing HTML
+document has a `lang` attribute, it is used to set the UI language in the
+player. Currently only supports English (`en`) and Swedish (`sv`).

multiplayer.js

@@ -1072,15 +1072,19 @@ input[type="range"]::-ms-track {
         /*
          *  Get the appropriate translation of key for the language
          *  stored in this._lang.
-         *
-         *  Since the keys are in english, the key is simply returned if
-         *  this._lang is 'en'.
          */
         i18n(key) {
+            // Since the keys are in english, the key is simply returned if
+            // this._lang is 'en'.
             if(this._lang == 'en') {
                 return key;
             }
 
+            // If we don't have a translation for the language, use english
+            if(!(this._lang in this._i18n)) {
+                return key;
+            }
+
             return this._i18n[this._lang][key];
         }