Dear Markus,
I am new to the forum and would like to help you. Can you clarify with me following points:
1. It seems something a module/plugin wants to translate your website, right? (yes/no)
2. Could you share with us with a screenshot, how the message looks like and where it appears?
I am not sure where the problem is yet, but I have one assumption.
a) you are using the google chrome browser, right?
b) if this is the case, I assume that the language detection plugin of Chrome is activated and always suggest to translate the website in your browser
c) open the website in another browser like Firefox or Internet Explorer, you will see that the website is not translated.
Hope I was able to give you some ideas where the problem might be.
Hasta luego senor 😉
P.S.: Read e.g. this post http://www.google.com/support/forum/p/Chrome/thread?tid=061eb9ea4b85cdf9&hl=en