Hey there spellbyte,
You can do one of the following:
- Perform a master reset from the menu (you might lose everything though)
- Try browse to the language folder and change the setting back
- Find someone that speaks german

Take a look at the manual, check out the language section. It tells you were abouts in the menu it is so you should be able to browse to it, even though you can't understand what it says.
www.sharp-phone.com/_assets/ downloads/en/030423_gx10i_e_link.pdf