It used to be so easy in Leopard via the information dialog one could set any application to start in the desired language. Some programs like Things or the Adobe Creative Suite are simply more effective in english. I want to have this feature back - but how…?
If you only want to do it occasionally, then look at the LaunchApp Plugin.
If you want to set it on a more permanent basis, run this script in a terminal window (a Terminal Window from /Applications/Utilities/Terminal):
defaults write $(mdls -name kMDItemCFBundleIdentifier -raw /Applications/Path\ to\ App.app) AppleLanguages "(de, en)"
Where you replace the path to your application (remember to escape spaces with a backslash) and the language codes with whatever you are looking for. After running this script, the application will always launch in whatever language you've specified first (assuming that language exists).
If you ever want to remove the preference, use:
defaults delete $(mdls -name kMDItemCFBundleIdentifier -raw /Applications/Path\ to\ App.app) AppleLanguages
Just to let you know, some applications, such as Adobe's Creative Suite, only install in one language. It's often not possible to switch like you can do with so many other OS X apps.
If interested, we also make another Language Switching app available on the MAS: http://itunes.apple.com/app/linguaswitch/id464896493?mt=12
All solutions to changing app language are based on what's available in the app bundle itself. They can't provide you with new languages by translating on the fly, as I notice some users of LinguaSwitch thought.