(Die/der/das are genders).
ドイツ語はぜんぜんわからない
(wa should be ga. wa means German is the subject that is doing the not-understanding.)
I do not.
Edit: After further reading は seems to work fine. In this case I think both work but が places greater emphasis on the german language being the thing not understood. で however was totally incorrect :P. But due to the tacit nature of informal Japanese I think the context already informed the reader who doesn't understand what.
Either way, は is fine in this sentence to the best of my understanding. It marks ドイツ語 as the subject, and thus the thing being understood.
If they demand dixyzx?
What if they demand custom declensions on verbs that involve them?
Another pretty elegant solution I've found if there's too much confusion amongst our team is to suggest people switch to learning Hungarian which doesn't have gendered pronouns. A few years of intense study is a small price to pay so that we can avoid the catastrophic mistakes of accidentally calling someone who's not in the room the wrong gender