As mentioned here:
https://www.mediawiki.org/wiki/Project:Requests/User_rights/Glaisher_2
If a user is trusted enough for Admin rights, he can be translateadmin, too, if he is sure he know, what he do.
Discussion at https://www.mediawiki.org/wiki/Thread:Project:Current_issues/Fwd:_Allow_sysops_to_self-grant_translation_administrator