English Translation
I can read German, but I can’t speak it.
FYI: you can replace ドイツ語は with ドイツ語を. The nuance of particle は in this context is to emphasize the difference of two (or more than two) things. Therefore, 私はドイツ語は読めるが話せない implies that I can read and speak a different foreign language, but my German is not so good as it.
If the listener is interested in discussing on German only, you should replace は with を because comparison is off-topic.