The tag is present in Lintian version 2.109.6. That is the most recent version we know about.
The package appears to ship locales for a language but uses an
incorrect 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).
When both standards define a language code for a given language, the ISO 639-1 code should be used (i.e. the two lettered code).
Lintian only knows about some commonly-mistaken set of incorrect locale codes.