[Bug 6495] Translator class should use english date

When "English" is set via libtranslate.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21844 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
GrampsLocale: Report the plugin when complaining

About not being able to find a translation.
Well, that's usually the source of this warning. With an unsupported
locale it will say "share".

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21845 4ae1f11a-8b86-4847-b8af-ab372f36d1fd