Since such functionality is in MediaWiki core, we should use the {{archived extension}} template, yes? :-)
Topic on Extension talk:PageLanguageApi
I'd wait until 1.29 is released. Otherwise, sure!
Since I'll probably forget to remove it once 1.29 is marked as stable (should be around Mid-2017), I'll just use the template right now. Cheers!
Hmm, why it's still translatable?
Because I haven't set the repository to read only yet, nor done anything except marking it obsolete. I'll do so when 1.29 is released. (And I might or might not know how to disable the translations for this extension). I'll look into that in due time, thanks for noting this!