Since DE Swedes the rule of “regular units should all speak their civ’s language” has been broken. But there still remain some units that need to have a different language voice lines than their civ:
Steppe rider and keshik (and Mongol scout): Mongolian
Fulani archer: Fulani
War wagon: maybe Czech/Bohemian? (it is the least “German” unit)
Russian horse archer: maybe they should speak Tatar? They look more like steppe Tatar people than Russian or cossack.
A lot of Indian units/natives/mercs: I am not listing all of them exhaustively but many should not speak modern Hindi, such as urumi or Maratha outlaws.
Also the huge amount of German mercenaries/natives could have some variant dialects. The same German lines are very overused.
Edit: most of the Balkan/southern European mercenaries and natives all speak Croatian. Maybe that could have a few varieties as well.