It seems many very user-visible applications need to (directly or indirectly) display output that come from PAM modules, such as the login and "Password: " prompts, as far as I understand it. This poses a big problem for internationalization, as this means that untranslated English strings are being very prominently injected into otherwise perfectly localized user interfaces. As a consequence, it must be made possible to translate the strings that PAM modules produce.
l10n will be supported by the next upstream PAM release.