1. install your language file from here
  2. about:config -> 'general.useragent.locale': 'de'
  3. restart Firefox
http://ftp.mozilla.org/pub/firefox/nightly/latest-mozilla-central-l10n/linux-x86_64/xpi/
http://ftp.mozilla.org/pub/firefox/nightly/latest-mozilla-central-l10n