For T197009: MCS should respect Accept-Language header for MW API requests we're writing some custom code into the service to take requests' HTTP Accept-Language headers and translate them into a single "best" value to pass to the uselang parameter.
However, this is not ideal, as whether or not MediaWiki can usefully respond to a given language is contextually dependent. A request for fr-CH;q=0.9,fr;q=0.6,pt;q=0.4 for a single-language-content wiki set to fr would work fine (as fr-CH falls back to fr), as it would for a single-language-content wiki on de (fr-CH eventually falls back to en, that's not fixable, so it'd come back in de.
On multi-content wikis, which right now means content variant wikis, asking for zh-Hant-MY would respond with zh-Hant when zh-Hant-SG might be a better match for the user's request language.
Something like uselangs would offload this code processing and picking of languages to MediaWiki, which is the right place for this decision.