ID3 allows saving a language for comment tags. Currently Picard is hard coded to always write "eng", and most of the files seen in the wild seem to default to "eng".

An exception is MediaMonkey, which allows the user to write "custom" tags. Those custom tags get stored to COMM frames with a description on "Songs-DB_Custom1", "Songs-DB_Custom2" etc. and language set to "XXX".

Picard not supporting this currently has two affects:

• Trying to create such a tag with Picard fails. $set(comment:Songs-DB_Custom1,test) will write a COMM tag with proper description, but language set to "eng".
• Loading such a file will basically duplicate the tag. The existing tag with lang=XXX stays and a new tag with lang=eng gets written.

See also https://community.metabrainz.org/t/help-with-script-to-send-release-type-to-mediamonkeys-custom-tag/442064