The tag is present in Lintian version 2.104.262. That is the most recent version we know about.
The name of this PO file doesn't appear to be a valid language
code. Any files in
debian/po ending in
.po will be
processed as translations by po2debconf for the language code equal to
the file name without the trailing
.po. If the file name does
not correctly reflect the language of the translation, the translation
will not be accessible to users of that language.
If this file isn't actually a PO file, rename it to something that
doesn't end in
.po or move it to another directory so that
translation merging programs will not be confused.