They had a former Bundesliga player announcing and he said they often use English as it is the most understood when they want to talk to the entire squad, especially if the squad had many foreign players.

But the Germans when they feel like being lazy will revert back to speaking German amongst themselves. I imagine there would not be a rule to speak German since many players would have zero clue what is being said.

Bayern speaks mostly English in the locker room.

Germany/Bundesliga is also atypical in that on the field during matches, you’ll hear German, English, Turkish and sometimes French.