All authentication plugins currently (I last checked in 1.6 beta 1) use a single language file for the strings. This is quite anti-modular since adding custom plugins requires changing the core file. I believe that each auth plugin should use their own language file for its strings and the common auth lang file for strings common to all plugins. Unless that is, the intention is that all auth plugins are meant to be distributed only by mainstream Moodle developers.
There are for example a few variations of the pop3 plugin. For example, I contributed pop3x (allowing to specify pop servers individually for each user) and somebody just posted instructions for pop3sub (allowing to require specific subdomain).