Acrobat comes with several language packages and runs by default with the language of the operating system. In this case however it doesn't accept serial numbers which are bound to a specific language. As it seems there are three work arounds (the first two are also mentioned by the Adobe support team):
The obvious one: Run OSX in the same language as Acrobat was purchased in. At least in my case not an option.
Use a program such as Language Switcher. I have not tried that but it probably does the same as option 3 below, only temporarily.
Remove the English language package (of course at your own risk): Open the Acrobat package, navigate to Contents/Resources and remove the folder 'en.lproj.'
[crarko adds: I haven't tested this one. I'd vote for option 2, I think. If you try option 3 be prepared to restore form backup/reinstall if you want to use English again.]

