[css-fonts-4] Add font-language-override descriptor #1203
Description
There should be a font-language-override descriptor in addition to a property.
There was a thread on www-style started by @jfkthame with a bunch of useful detail and discussion in it, but I couldn't find a github issue.
I'm tagging this as css-fonts-4 under the assumption that the font-language-override property is likely going to get deferred from css-fonts-3 because it has only one implementation (I think). If that's not the case, we should check implementations of the descriptor as well, since font-language-override as a descriptor is implemented in Gecko.