language: allow lang-region as variant prefix

Latest IANA language repository file introduced a variant that has
a prefix of the form language-region. This CL adds support for this.

Updated tables will be included in a separate CL.

Change-Id: Id7e85ca3e76d466ec89f0b8e4ce9a98871f2d5bf
Reviewed-on: https://go-review.googlesource.com/13452
Reviewed-by: Nigel Tao <nigeltao@golang.org>
1 file changed