While I was living in Spain I had this same problem myself. I am sort of fluent, but it didn't get the jokes and the personality across. Unfortunately spaniards hardly speak any other language, so I had to up my spanish, which was a good thing in the end.
I'd say you could mix a little of both languages. Approach in english, it creates interest, but have some of the convo in german as well, it shows respect for the country and the fact you're not some dumb American who can't say a word in another language.
You will simply have to avoid word games and rely on non-verbal seduction. Maybe 60 years of challenge can help you here. If you understand enough of their dialect, you can let her talk in german, german dialects are a bitch and maybe she can make an effort to speak a more standard language.
Viel Spass
