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

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.