Returns a well-formed IETF BCP 47 language tag representing this locale.

If this Locale has a language, country, or variant that does not satisfy the IETF BCP 47 language tag syntax requirements, this method handles these fields as described below:

Language: If language is empty, or not well-formed (for example "a" or "e2"), it will be emitted as "und" (Undetermined).

Country: If country is not well-formed (for example "12" or "USA"), it will be omitted.

Variant: If variant is well-formed, each sub-segment (delimited by '-' or '_') is emitted as a subtag. Otherwise:

if all sub-segments match [0-9a-zA-Z]{1,8} (for example "WIN" or "Oracle_JDK_Standard_Edition"), the first ill-formed sub-segment and all following will be appended to the private use subtag. The first appended subtag will be "lvariant", followed by the sub-segments in order, separated by hyphen. For example, "x-lvariant-WIN", "Oracle-x-lvariant-JDK-Standard-Edition".
if any sub-segment does not match [0-9a-zA-Z]{1,8}, the variant will be truncated and the problematic sub-segment and all following sub-segments will be omitted. If the remainder is non-empty, it will be emitted as a private use subtag as above (even if the remainder turns out to be well-formed). For example, "Solaris_isjustthecoolestthing" is emitted as "x-lvariant-Solaris", not as "solaris".

Special Conversions: Java supports some old locale representations, including deprecated ISO language codes, for compatibility. This method performs the following conversions:

Deprecated ISO language codes "iw", "ji", and "in" are converted to "he", "yi", and "id", respectively.
A locale with language "no", country "NO", and variant "NY", representing Norwegian Nynorsk (Norway), is converted to a language tag "nn-NO".

Note: Although the language tag created by this method is well-formed (satisfies the syntax requirements defined by the IETF BCP 47 specification), it is not necessarily a valid BCP 47 language tag. For example,

   new Locale("xx", "YY").toLanguageTag();

will return "xx-YY", but the language subtag "xx" and the region subtag "YY" are invalid because they are not registered in the IANA Language Subtag Registry.

Returns: a BCP47 language tag representing the locale