For example in Egyptian Arabic, we aren't showing the reply icon, even though that language is using standard Arabic messages via the language fallback mechanism:

https://arz.wikipedia.org/wiki/%D9%88%D9%8A%D9%83%D9%8A%D8%A8%D9%8A%D8%AF%D9%8A%D8%A7:%D8%B5%D8%A7%D9%84%D9%88%D9%86_%D8%A7%D9%84%D9%85%D9%86%D8%A7%D9%82%D8%B4%D9%87?dtenable=1

Rather than explicitly listing all current known fallbacks, we should do the following:

• If a language is not found in the "reply_icon_languages" list (currently ar/zh/yue/ja/nod), get the fallback language list for that language
• Check each fallback, and if it is found in the list, use the icon

As we may end up in a situation where a variant doesn't need an icon, even though one of the fallbacks does, we should convert the list to a dictionary and allow explicit true/false values. So, for example, if an Egyption Arabic translation was added and it was significantly longer than standard Arabic, we could explicitly set "arz": false to avoid it falling back to the true value from standard Arabic.

Testing

• Talk pages will show the reply arrow icon when using varaints of ar/zh/yue/ja/nod, e.g.
  • ?uselang=arz
  • ?uselang=zh-tw