FULL PRODUCT VERSION :


A DESCRIPTION OF THE PROBLEM :
When an instance of Locale has empty language, toLanguageTag() needs to add "und", but except when private use field is available.

For example -

    String tag = Locale.forLanguageTag("x-elmer").toLanguageTag();

The code above should return "x-elmer", but it actually returns "und-x-elmer".


REPRODUCIBILITY :
This bug can be reproduced always.