The validator should be stricter with language codes

[bfabio]

checkLanguageCode should be stricter and check for valid BCP47 tags only, as defined in the standard

[bfabio]

Go's language.Parse() is lenient and allows non-BCP47 language tags, this probably means we need to switch the parsing library altogether to a stricter one, which I don't know if it exists.

Putting on hold for now.