Development:Places to search for lang strings
From MoodleDocs
Moodle has a mechanism that allows a number of places to be searched (in order) to find language strings or help files. This enables help to be packaged with optional plugins and avoids the step of having to copy the language files over to the language directory when a plugin is installed. This page provides some information about this mechanism which might be useful to plugin developers or those wishing to add a completely new pluggable feature to moodle.