The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
The package appears to ship locales for a language but uses an
unknown locale code as a subdirectory of
This usually results in users of the intended target language not
finding the locale. The language codes used in the locale directories
are those from the ISO 639-1 and ISO 639-2 standards, not those
usually used as TLDs (which are from the ISO 3166 standard).
It is possible that the language code was mistyped or incorrectly guessed from the language's or country's name.
Please refer to http://www.loc.gov/standards/iso639-2/php/code_list.php for details.