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
1In addition to following the directions below, you may want to contact the developers of those applications to let them know that their localization needs improvement. – s4y – 2009-10-05T22:23:27.203
1It's a new 'feature' of Snow Leopard, that you can’t change the default language anymore via the information dialog. :( – patrick – 2009-10-08T10:02:54.580