Documentation for language form type field is misleading

[cblegare]

As stated in this documentation page

The "value" for each language is the Unicode language identifier (e.g. fr or zh-Hant).

which is wrong. Symfony uses some custom standard using underscores, not dashes, for languages.

See also issue #2468

[stof]

documentation issues should be reported in the symfony-docs repo, not in this repo.

[xabbuh]

see symfony/symfony-docs#4251