https://www.cups.org/doc/translation.html says:

After you have translated all of the templates, add the locale to the LANGUAGES variable in the Makedefs file

But https://github.com/apple/cups/blob/master/Makedefs.in#L78 says:

LANGUAGES = @LANGUAGES@

So maybe that text does not apply anymore and should be deleted if it is not necessary for the translators to add a locale to LANGUAGES.