Content Languages: Prevent saving an incorrect language tag when saving a Content Language

[infograf768]

Create a Content Language. You can save an incorrectly formatted Language Tag, i.e. a tag which does not obey to the simple rule:
2 or 3 lowercase letters + dash + 2 Uppercase letters
Example: fr-FR, ckb-IQ, etc.
The field accepts anything....

This patch throws an error when the format is wrong.
After applying, try to save anything different from the rule above.

For 3.4.1

[Fedik]

test
works good,
now I do not able to save blabla 😄

[Hils]

@test
Confirm that, after adding patch, unable to save fvfvfv (an incorrect ISO Country code).
(3.4.0 RC1)

[zero-24]

RTC based on testing. Thanks!

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