> an exclusively bilingual cast,
Not really. First off, most Germans seem to know English passably well. Even if one doesn't know English well, one can still memorize/recite dialog. There isn't that much dialog in a show. And lastly, one can always dub the ones who can't do it, as a last resort.
> dialog that works equally well in both languages,
A requirement anyway if it is being dubbed.
> and would drastically increase the production cost
Increase it, sure, not so sure about drastically. Why spend a lot of money on the production (as is obvious in Babylon Berlin) only to make it sound crappy to the biggest market (English speaking)? An English soundtrack makes it accessible to most of the world.