Re-add code and und language tags
_The documentation is not available anymore as the PR was closed or merged._
This PR fixes the removal of 2 language tags done by:
- #4882

The tags are:
- "code": this is not a IANA tag but needed
- "und": this is one of the special scoped tags removed by 0d53202b9abce6fd0358cb00d06fcfd904b875af
  - used in "mc4" and "udhr" datasets