1. I´m using the german language for the gallery and for the board. I want to keep it in german. Is 5.1 already supporting it?
German is included
2. I want to keep the data (pictures, albums and so on)...what should i do to keep em after the switch to standalone?
You would need to copy all files to the new server.
3. Should i reinstall the gallery to 5.1 but keep the album / picture path and the sql db?
You would move everything to the new server or domain and repoint server paths and urls in the settings mysql table with something like phpmyadmin. You can then do a regular upgrade
4. How can i get rid of the board lookalike...i now there were something with a header / template but i just forgot it. ;-)
You would turn forum style integration to off and in the old header-inc.php you need to comment out all the vb
5. is i possible to run this Mobile 2 Gallery Addon with 5.1? In my current version it was not working
yes there is a support mobile mod for 5.1