Localization of XUL applications in general and Mozilla applications in particular are done by a number of different file formats. Independent of the format, the Mozilla architecture does not provide fallback strings at runtime. The compare-locales library and the calling scripts provide a way to check a given localization for completeness.
compare-locales needs to be installed, and it requires that you have Python installed on your machine. The easiest way is to just
easy_install -U compare-locales
compare-locales gathers the directories to compare from an ini file, usually found in $APP/locales/l10n.ini. You pass both the path to the ini file and the parent dir of the localizations as first arguments, followed by the locale codes of the locales you want to compare. If you want to test the localization for de, run
compare-locales browser/locales/l10n.ini ../l10n/ de
As of version 0.5, the l10n directories don't need to be in a bound place relative to your mozilla dir. This works on the CVS branches as well as on mozilla-central.