I had asked on wiki because I just assumed it had been deliberately overwritten locally, and I wanted to find out more. (The link correctly works elsewhere AFAICT.)

"Help center" link on the sidebar links to https://commons.wikimedia.org/wiki/Special:MyLanguage/Help:Contents for me.

It is likely a translation problem (you should see it if you change your language to Italian, for example). I tweaked https://commons.wikimedia.org/wiki/MediaWiki:Helppage/it but that didn't change anything (yet?).

OK, I see it when I change my UI language to Italian. The code is working as expected. I am changing this to a site request.

What happens here is that 'helppage' is included in $wgForceUIMsgAsContentMsg at Commons. This means that the message will be retrieved from the user's interface language instead of the content language. The reason why your edit to MediaWiki:Helppage/it hasn't taken effect is because the sidebar's content is cached for a day and so it will be regenerated tomorrow since you made the edit today.

If you feel that this is the not the "proper" behavior, request the configuration to be changed as per the usual site requests process. If not, please close this task.

I'm confused. Do we agree that all users looking for "help" on Commons should land on a help page on Commons itself, about Commons?
What I expect is that if I click on that link (with lang set as Italian) I land on https://commons.wikimedia.org/wiki/Help:Contents/it , and that if that page didn't exist, that it'd fall back to https://commons.wikimedia.org/wiki/Help:Contents.

In T7925, helppage has been added to wgForceUIMsgAsContentMsg.

As it now contains an universal value (Special:MyLanguage/...), we can remove it.

Change 306255 had a related patch set uploaded (by Dereckson):
Fix UI l10n for Help page link on commons.wikimedia.org

https://gerrit.wikimedia.org/r/306255

Change 306255 merged by jenkins-bot:
Fix UI l10n for Help page link on commons.wikimedia.org

https://gerrit.wikimedia.org/r/306255

Mentioned in SAL [2016-08-23T19:04:02Z] <thcipriani@tin> Synchronized wmf-config/InitialiseSettings.php: SWAT: [[gerrit:306255|Fix UI l10n for Help page link on commons.wikimedia.org (T143564)]] (duration: 00m 47s)

I want to point out I just brought up what I believe is an issue. If this requires further discussion, review, involvement of Commons users etc., by all means please make sure these steps happen :)

The Commons community implicitly required it when they switched to Special:MyLanguage/ the value.

So, it has been deployed, but we've still former values in cache. This is rather heavily cached, so a manual purge should occur (we bumped on a "MemCached error" doing that, T143722 reported about that).

I don't feel like I have received an answer to my underlying question on Commons :)
If sending some users to the actual page, and others to an unrelated one on a completely different site is indeed what was desired, then there's no need to do anything: I just want to understand how this is desirable, because it doesn't look like it to me.