I recently ran into the problem with Gallery2 2.2.6. Changing the language of Gallery2 to German in the settings didn't work; everything was still in English.
"gettext" was installed correctly (as suggested by the Gallery2 wiki).
I also looked through the following sites but there was no solution to my problem.
In gallery2/modules/core/locale/de/LC_MESSAGES/modules_core.mo says:
The following command shows which locales are installed on the system:
$ locale -a
As you can see de_AT (German Austria) is installed but de_DE (German Germany) is not.
I ran "dpkg-reconfigure locales" as root and added the following locales:
After restarting Apache with "apache2ctl restart" Gallery2 was finally in German. 🙂