For comparison, Translate adds by itself a preference containing a list of languages, for the assistant languages features.
Bug 41686 should probably be implemented as part of CleanChanges, used for the same purpose on Translate wikis.

I don't understand this bug. The language in preferences is currently only used for the site interface, as I understand it. (Is this correct?)

What would allowing more than one language to be selected accomplish?

(In reply to comment #2)

I don't understand this bug. The language in preferences is currently only
used for the site interface, as I understand it. (Is this correct?)

No, it's also used for content. For example, https://meta.wikimedia.org/wiki/Meta:Language_select will filter the category descriptions on a page like https://commons.wikimedia.org/wiki/Category:Earth .

You can override it to show all languages, or an arbitrary one. But it defaults to your preference language.

Same for https://www.wikidata.org/wiki/Q2 (Earth at Wikidata). All the labels are localized, using your preferences by default.

What would allowing more than one language to be selected accomplish?

I gave clear examples in the report, such as filtering your watchlist and recent changes. Look at https://www.wikidata.org/wiki/Special:RecentChanges for example. It would be nice to only see the label changes for languages in my preferences.

Actually, Language select may default to your browser language. Either way, the point about the watchlist, recent changes, etc. applies.

(In reply to comment #3)

I gave clear examples in the report, such as filtering your watchlist and
recent changes. Look at https://www.wikidata.org/wiki/Special:RecentChanges
for example. It would be nice to only see the label changes for languages in
my preferences.

Obviously I didn't find the examples in comment 0 very clear; that's why I asked for a clarification. Thanks for the links.

You should file separate bugs about each individual component that you think should have a language selector (Special:Watchlist, Special:RecentChanges, etc.). This bug would be a blocker or a dependency of those bugs.

Bug 41686 (listed in the see also) is about watchlist and RC filtering. That can work even if there's only one language, but it's more useful to multi-lingual people if this is also implemented