Filters cannot have language files (a lang/ subdirectory).
This is inconvenient when creating contrib filters as it means you cannot create a filter that is entirely contained within one directory.
I intend to make filters work in the same way as everything else: if you call get_string('whatever','filter_something'), it then looks for filter_something.php in filter/something/lang/en_utf8 (as well as in lang/en_utf8).
This is a one-line addition to existing code in moodlelib which was designed for this purpose, so should work across the board without causing problems.
I searched code for get_string(.*filter_ and didn't find any, so this should not change any existing behaviour.