[RFC] [Validator] Suggest to apply `\Locale::canonicalize()` before validating with `Locale` constraint

[phansys]

It would be nice to have a note about \Locale::canonicalize() within the Locale constraint documentation, since many times locales are expressed using other "uncanonicalized" formats. By instance, a common use case is to infer the locale from javascript, where navigator.language returns values like "en-US" instead of "en_US"; making the Locale constraint to fail.
The canonicalization process is explained at ICU project.

[phansys]

As a side note, and to add more reasons for this suggestion, the referenced article at Wikipedia for ISO 3166-1 alpha-2 doesn't mention anything about the separator to use between the language and country codes.

[fabpot]

Will be a feature implemented in 4.1 (see referenced PR).