See also T161517

It would be better if the browser settings (Accept-Language) and/or geolocation would be considered.

I strongly suggest using the Accept-Language header. I expect this to cover ~100% of users/browsers, and that's how this is intended to be used. Also, I do not want my UI to suddenly be in Icelandic just because I'm quickly looking something up while being on vacation in Iceland.

To let people who aren't logged in change the language, set $wgULSAnonCanChangeLanguage to true.
To enable language detection, set $wgULSLanguageDetection to true.

If you change the former but not the latter, it will default to English but people who aren't logged in will be able to change the language manually. If you change the latter but not the former, it will default to the browser language and people who aren't logged in won't be able to change it manually.

They're both currently disabled in wmf-config/CommonSettings.php in the operations-mediawiki-config repo for performance reasons. Doing that is understandable for the monolingual wikis, where using the wiki language as the interface language is a reasonable assumption, but I really think it should be changed for (supposedly) multilingual wikis.

According to the Pageviews Analysis tool, Commons and Wikidata are only a tiny amount of the traffic compared to the biggest Wikipedias (link) and Wikidata has more than the rest of the multilingual wikis I could think of combined (link).

Commons has had a script to allow changing the interface language for many years. It adds a banner to the page suggesting a language to switch the interface to, adds a dropdown menu to the sidebar and links from other projects often include the uselang parameter by default. I've been seriously considering adding that script to Wikidata as well for a while now. It would be much better if it could be done without using a hacky script like that though, and if it hasn't been causing problems on Commons after all this time, is there a reason why we can't change those settings for multilingual wikis?

Thank you, @Nikki, I was not aware that this functionality already existed, and we just need to switch it on. This is great news! \o/