[selectors][css-text] Define "content language" in one place

[zcorpan]

From #869 (comment)

'hyphens' uses https://drafts.csswg.org/css-text-3/#content-language
':lang()' uses https://drafts.csswg.org/selectors/#language

I think there should only be one concept defined here that css-text and selectors should both hook into.

[annevk]

See also #2260. Leaving this up to the DOM (or equivalent) makes more sense.

[fantasai]

This was fixed in 9eed0f9 actually, I just typed in the wrong issue number. :(

@annevk The Selectors/CSS Text specifications already defer to the document language on this.