Adding ULS and Compact links tags. It won't necessarily be fixed by them, but it's very relevant to them, and was requested numerous times in their context.

Another similar issue: T135374.

Also, this must be global to all devices—desktop, mobile web, mobile apps.

Currently, language preferences are often stored not necessarily as preferences, but also in a variety of other ways, for example cookies, localStorage and Babel boxes. Yes, it's a mess :(

Is this something that gets much easier with the completion of T16950? Is it enough easier that it's worth holding out for the completion of that task, or is this important enough to create a workaround if T16950 is too far from being complete?

@Amire80: do you think it should be set only by a special UI in ULS, with a checkbox to set the global language as @tarlocesilion suggests, or do you think it should also be integrated with Special:Preferences? If it's on Special:Preferences, what would that look like?

Discussing the UI feels a bit premature to me when there is no clarity when and how the support can be implement in the backend.

I also wonder about the requirement about mobile apps: do they currently use the language setting of the wiki or separate language setting? Is that even in scope for this bug?

Would global user preferences work instead?

In T120385#3827441, @TerraCodes wrote:

Would global user preferences work instead?

Not "instead" :)

Global user preferences is precisely the right solution for this and for many other issues.