IMPORTANT: due to a drive failure, as of 13-Mar-2021, the Mercurial repository had to be re-mirrored, which changed every commit SHA. The old SHAs and trees are backed up in the vault branches. Please migrate to the new branches as soon as you can.
This lets clients know that the field is a language field and should be
in RFC 5646 format.
Field validation code in util.dataforms left for future commit.