We aren't deliberately doing this but since my husband's first language is Dari Yasin and Imran are exposed to 2 languages. They also are exposed to a smattering of Arabic and Yasin has a few Spanish words as well.
It doesn't cause any developmental delays but if they are at a stage where they could be expected to have 300 words they are likely to have 150 in one language and 150 in another not 300 in each.
Generally bi-lingual babies don't start talking any later than other babies so if you're worried about your babies speech don't delay in consulting someone on the grounds that they are bi-lingual. Of course babies reach milestones in thier own time so delayed speech isn't a reason for panic attacks. In my Yasin's buddies group we have 2 bi-lingual babies. One of them is a really quick speaker and Yasin is lagging behind a bit.
Since differant languages have differant sounds being bi-lingual stimulates differant parts of the brain in the same way as exposure to music does.
Also if you learn a language while you are young if you want to re-learn it later (or a differant language) it will be easier for you than someone who hasn't been exposed to it previously.
I wouldn't worry too much about your baby picking up your mistakes in German. lots of people don't speak thier first language perfectly and for communication purposes its amaxing how much we can understand even when people have very poor language skill.