In general you should set the LANGUAGE and LANG environment variables before starting GnuCash. To do this you need to open a terminal and run the following command:

LANGUAGE=ll_LL LANG=ll_LL gnucash

ll_LL is the locale you want to run GnuCash with (de_DE for Deutsch, it_IT for Italian etc.)

	Note
	On some systems (e.g. Ubuntu) the encoding could be part of the locales name, like ll_LL.UTF-8. You can use these names as well.

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

defaults write $(mdls -name kMDItemCFBundleIdentifier -raw /Applications/GnuCash.app) 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.

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

defaults delete $(mdls -name kMDItemCFBundleIdentifier -raw /Applications/GnuCash.app)

If you are running GnuCash 2.4.0 (or newer) on Windows, you can set the interface language by editing the file environment with a text editor (e.g. Notepad). By default this file is installed in c:\Program Files\gnucash\etc\gnucash. Change this file such that the last few lines are:

# If you wish GnuCash to use a different language, uncomment the two parameters
# below and set LANG to your preferred locale
LANG=ll_LL
LANGUAGE={LANG}

ll_LL is the locale you want to run GnuCash with (de_DE for Deutsch, it_IT for Italian etc.)