feat: add isISO6391 decorator for language codes

[RobinvanderVliet]

Description

Add isISO6391 validation decorator for ISO 639-1 language codes.

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified)
• documentation added or updated
• I have run the project locally and verified that there are no errors

Fixes

fixes #2045

[braaar]

Please fix the test so that we can proceed with this

[RobinvanderVliet]

@braaar I have added the new decorator to the exports. Now the tests run successfully on my machine.

[viniciusamc]

hey @braaar, what's the status of this PR?

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.