You may not need to "insist" insist either, ie., no need to explicitly ask or make it a problem if German colleague speaks English :) Conversation is a two-way

You may not need to "insist" insist either, ie., no need to explicitly ask or make it a problem if German colleague speaks English :) Conversation is a two-way street and you both have agency. They reply in English... you just hit them right back with German! (maybe with a question to clarify if they got it, in case your accent is really strong)