No, it's fine as is. I think you have は and が backwards but neither of them imply ドイツ語 is "doing something" even though it's the subject of the sentence. It's a pro-drop language so meanings that don't make sense are just excluded.

>I think you have は and が backwards

I do not.

は is not the "subject", it's the "topic", which is different.

I was split between で or は, forgot all about が. Funnily enough I think omitting the particle altogether would have made more sense.

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.

Yes, people will understand what you meant if you used wa.

I don't think it's a matter of people simply understanding what I meant to say, it's grammatically correct from what I have gathered.

が marks the subject, and the subject of わかる is the thing that's being understood, not the thing that's doing the understanding. (Maybe you meant subject in the non-grammatical way? It's confusing.)

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.