Returns a name for the locale's language that is appropriate for display to the
user.
If possible, the name returned will be localized for the default
DISPLAY locale.
For example, if the locale is fr_FR and the default
DISPLAY locale
is en_US, getDisplayLanguage() will return "French"; if the locale is en_US and
the default DISPLAY locale is fr_FR,
getDisplayLanguage() will return "anglais".
If the name returned cannot be localized for the default
DISPLAY locale,
(say, we don't have a Japanese name for Croatian),
this function falls back on the English name, and uses the ISO code as a last-resort
value. If the locale doesn't specify a language, this function returns the empty string.
Diagram: Locale