Allow engines to ship data for locales in Accept-Language #1010
Description
As a follow-up to #588 and #780, we are now specifying that the available locales ought to be derived from the User Agent, and we left some room to explore how we can support additional locales.
I've floated the following idea, and it seems like it may be workable: allow the available locales list to derive not only from User-Agent but also from Accept-Language aka navigator.languages.
In order to avoid the fingerprint, it should be the case that, given any User-Agent/Accept-Language combo, the set of available locales is always the same. So, any particular User-Agent should either always or never change the set of available locales based on the Accept-Language, and it should always do so in the same way.
I've gotten questions about whether this approach is impacted by efforts to reduce the fingerprintability of Accept-Language. I think it is not, because we are allowing engines to add additional locale data based on the Accept-Language, and if the Accept-Language is reduced, then the engine is reduced in the amount of data that it can add. So, these are two disjoint problems.