Can I change the default language of a application / program in Snow Leopard?

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

One could manually remove all the other languages of an application by right-clicking it > show package content < and remove all unwanted languages.

But this is far of from an elegant solution. :(

There’s now an application which handles the task elegantly; you can even set the language permanently: Language Switcher.

Update: There is now another application which is available in the MAS: App Language Chooser.

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.