RFC 1766 was obsoleted by RFC 5646 which allows 3-letter codes and
many other subtags. XML formats (xml:lang etc.) use this hyphen-delimeted format.

Glibc uses ISO 639 language (2- and 3-letter) codes, optionally followed by an
ISO 3166 (2-letter) country code, followed by modifiers (like "sr@Latn";
compare to RFC 5646 "sr-Latn").

DECISION
========

Because RFC 5646 format is more difficult to parse, because YAML libmodulemd
already uses glibc format, and because comps group also use glibc format
(despite written in XML), XML module format will use glibc format.

