This is one of those threats that is always looming in the wings. My company has English as the official language, but that memo seems to have missed some of my swiss workmates that only have to deal with the domestic market and of course my fench colleagues that simply refuse to admit that there could possibly exist another international language. C'est la vie, n'est pas?
But seriously, maybe it is a little early to pack up the pictures and plants. Being the deparment head, couldn't you delegate the grunt work of processes, training modules and documents to other members of your team? Maybe this would provide you with the time needed to fortify your Deutsch or if your area is in IT, then it would be justifyable to keep your process, tm.... in English.
Worst case scenario, argue the point to have the basic overview in both German and English (we have ours in 3 languages) and keep the bulk in English. Point to bring up:simply for the sake of time needed to translate and the amount of external data arriving on a daily basis, it would require an excessive amount of resources to translate into German(yada,yada,yada).
In this case (specially if the new head doesn't really have a clue of what you do) your power to debate and persuade will be essential.
Good luck and feel him out in order to know how you have to approach him.
Don't forget, someone there thought you capable enough to give you the job and any boss is a fool to give the chop to anyone who was considered capable prior to his arrival.
Lundi