[AzazelSensei]

Summary:

The mismatch notice help text said mul comes from ISO 639-2. GTFS language fields are BCP 47, so I updated that wording on the notice and the validator class.

Fixes #1987

Expected behavior:

Help text for feed_info_lang_and_agency_lang_mismatch now says language codes follow BCP 47.

• Run the unit tests with gradle test to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[skalexch]

@emmambd I raised a related issue here.

[jcpitre]

Thanks for the contribution!

[welcome]

🥳 Congrats on getting your first pull request merged!