Fix order of loading gettext catalogs

Load the "poedit" catalog as the last one, because it should take
priority over wxstd translations.
Handle language codes with UN region code properly

Instead of disallowing such locales completely in the list of languages
(as 90aed6cd did), allow them in the list and properly validate them in
the regex. Apparenly, es_419 in particular is commonly used with
gettext, so this is the right thing to do.