caos: Use IETF language tag representation instead of Posix locale
caos: Use IETF language tag representation instead of Posix locale

The Web mostly uses the IETF/W3C notation for language tags, and we
actually only want to identify the language itself, whereas the locale
string also includes encoding and modifiers, which is not applicable in
this case.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb