languages: Use a more broadly compatible locale codeset suffix

At least OpenBSD's libc doesn't accept 'utf8' as a locale codeset
suffix but does accept 'UTF-8'. Since GNU libc accepts both suffixes
let's use the one which works on a broader set of systems.

Thanks to Stefan Sperling and Antoine Jacoutot.

https://bugzilla.gnome.org/show_bug.cgi?id=710412