Use IETF BCP 47 language tag instead of ISO 639-1

[baptiste0928]

The library actually uses ISO 693-1 languages identifiers, which are very simple languages identifiers (2-letters code), but does allow for precise identifiers. For example, en-US is not a valid ISO 693-1 identifier.

IETF BCP 47 is a standardized code used by many standards such as HTTP and HTML. This format is compatible with ISO 693-1 identifiers. As for actual identifiers, the library will not provide a way to validate if a specific language tag exists, but will rather focus on parsing and canonization.