Returns a name for the locale's language that is appropriate for display to the
If possible, the name returned will be localized for the default
For example, if the locale is fr_FR and the default
is en_US, getDisplayLanguage() will return "French"; if the locale is en_US and
DISPLAY locale is fr_FR,
getDisplayLanguage() will return "anglais".
If the name returned cannot be localized for the default
(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.