By default, Some Applications will select the first available translation from the list in your language list (depending on what version of OSX you’re using).

If you want to use a different translation from the one that is automatically selected, you can run the following in

defaults write -app AppName AppleLanguages '(de, en)'

(Use whatever language codes you want, replacing Deutsch and English. It won’t work if there isn’t a translation file for the language you want.) Some versions of OS X don’t accept -app AppName and emit Can’t determine domain name for application AppName. In that case use namespace of app in place of -app AppName.

If you want to unset it (that is, return to using the system settings), run this:

defaults delete -app Gnucash AppleLanguages

If you have questions, suggestions or any feedback can write in the blog comments or email me