As an elementary ESL teacher and university-trained linguist (in second language acquisition), it is wonderful that your daughter is going to a German-speaking preschool. She will soak up the language so quickly and the little neurons in her brain will connect so much faster, thereby giving her more skills and abilities to learn additional languages easier, become more creative, etc, etc.
I am an American, living in the Netherlands. My ds, who is almost 1 year old, hears only English at home with me, Dutch at the babysitter's house and Dutch when his father visits. I am very anxious and curious to find out what his first word is going to be, in which language!
Additionally, I think you will find that you are far more anxious about the situation than your daughter is. You will really need to try to "control" that because she will feed off of it. She should not have any problem AT ALL to adjusting to a German speaking school. She will probably have more of a problem adjusting to just being in school than anything else.
Here are a couple of great resources for you..the more you educate yourself about bilingualism, the less anxious you will be:
"The Bilingual Family: A Handbook for Parents" by Edith Harding and Philip Riley, published by Cambridge University Presshttp://www.cal.org/resources/faqs/RG..._children.htmlhttp://www.multilingual-matters.com/
They have a great newsletter, "The Bilingual Family Newsletter"
You are giving your child a wonderful gift. Good luck!