Returns a well-formed language tag representing this locale.
Note: If the language, country, or variant fields do not satisfy BCP47 language tag syntax requirements, they are omitted from the result. For example, using the constructor it is possible to create a Locale instance with digits in the language field, or only two characters in the variant field. Since these are not well-formed BCP47 language tag syntax, they cannot be expressed in BCP47. Since such 'legacy' locales lose information when converting to BCP47, it is recommended that clients switch to conforming locales.
Note: Underscores in the variant tag are normalized to hyphen, and all fields, keys, and values are normalized to lower case.
@return a BCP47 language tag representing the locale.
@draft ICU 4.2
@provisional This API might change or be removed in a future release.