Added german as selectable in options

Also added a comment in i18n that refers to how to add a language to the game settings menu to actually select it.

src/system/settings.ts

@@ -184,6 +184,10 @@ export function setSetting(scene: BattleScene, setting: Setting, value: integer)
                 label: 'French',
                 handler: () => changeLocaleHandler('fr')
               },
+              {
+                label: 'German',
+                handler: () => changeLocaleHandler('de')
+              },
               {
                 label: 'Cancel',
                 handler: () => cancelHandler()