What do you expect to happen if you move a translated page? It would lose its connection to the main page. What to do with the translation unit pages (Translations: namespace)? Would the moved page become freely editable? What would happen with the original title? Turning into a redirect is not an option, since it has to use the translation page template, which is probably not a redirect.

By the way, it doesn’t work the other way either AFAIK, i.e. I can’t move Project:Hilfe to Project:Help/de if Project:Help is a translatable page (even though that’s probably a more common use case: as a translation administrator, I find Project:Hilfe after I’ve moved Project:Aide to Project:Help/fr, Project:Nápověda to Project:Help/cs etc. and have already marked Project:Help for translation; now I have to remove the English page from translation, move the German page and re-mark the English page for translation).

Hmm I think you're right. I confused *deletion* of a translation with *moving* of a translation page. I recently had to rename a language code, where this was needed. So this is not a bug, but a new feature request.

I reflected to this as a feature request, so my questions still stand. However, if you’re willing to limit this feature to only changing the language code (instead of renaming to arbitrary titles), they’re probably easier to answer—the connection to the main page isn’t lost, the page doesn’t become freely editable, and the translation units can simply be also moved (because of the latter, it would make sense to limit the ability to do so similarly to moving root pages and probably commit the moves under FuzzyBot’s name).