When the graphical user interface is using a wrong language, it may be caused by corrupted Preferences files.
Try, if removing GarageBand's preference files will help. Remove these files from your User Library to a folder on your Desktop:
~/Library/Containers/com.apple.garageband10/
~/Library/Preferences/com.apple.garageband.plist
~/Library/Caches/garageband
Quit the AppStore and GarageBand, then remove the files to a folder on the Desktop and restart the computer, before trying again to open GarageBand.
You user library may still be hidden, as is the default in Mavericks: To open your hidden user library:
- Select the "Home" folder icon (the little house) in the Finder's sidebar and press the key combination ⌘J to open the "view options".
- Enable "Show Library Folder".
- Then open the Home folder and open the Library folder inside and navigate to the Preferences, Caches, or Containers folder. Remove these folders completely - don't leave anything inside: ~/Library/Containers/com.apple.garageband10/,
~/Library/Caches/garageband