[css-fonts] Specify font-language-override as a @font-face descriptor

[nox]

https://bugzilla.mozilla.org/show_bug.cgi?id=1355364
https://lists.w3.org/Archives/Public/www-style/2017Feb/0028.html

[litherum]

We resolved during the Tokyo F2F to do this. I believe there is an outstanding pull request for me to take.

[litherum]

Done in e18ba05

[litherum]

Oh wait, this wasn't done yet.

[litherum]

e0f8bba