The current implemented methods Locale.filter/lookup() and Locale.filterTags/lookupTag() return lowercased language tags. The language tags should be treated in case-insensitive manner, so the tag "mn-Cyrl-MN" is not distinct from "MN-cYRL-mn" or "mN-cYrL-Mn" (or any other combination) and each of these variations conveys the same meaning, but RFC 5646 recommends that the language tags should follow some convention.

   These conventions include:

   o [ISO639-1] recommends that language codes be written in lowercase
      ('mn' Mongolian).

   o [ISO15924] recommends that script codes use lowercase with the
      initial letter capitalized ('Cyrl' Cyrillic).

   o [ISO3166-1] recommends that country codes be capitalized ('MN'
      Mongolia).

Changing the the existing implementation is not backward compatible and may break some applications. So, a new method should be introduced in the Locale class which can take an input string of language tags and convert their case convention as per RFC 5646 standard.