When you click the "Aあ" icon on the top-right corner of https://wikimediafoundation.org/wiki/Home, you will see this message.
Display language: English (same as content)
Log in to select a different language for menus.
However, this instruction will not work for most visitors who cannot log in there.
It appears that this is ultimately dependent to T58464. In the meantime, there could be several workarounds:
1. Replace "Log in to select a different language for menus" with "We are sorry, language selection capability is currently disabled" or something to the effect. Or perhaps hide the "Aあ" icon. Any logged-in user of wikimediafoundation.org with MediaWiki-namespace editing privilege can do this.
2. Adopt the JavaScript hack of Commons mentioned in T161517. (The main concern against the JS was scalability which I believe less of an issue for wikimediafoundation.org.) Any logged-in user of wikimediafoundation.org with MediaWiki-namespace editing privilege and JS knowledge can do this.
I'd suggest that #1 should be done quickly. And then it can be replaced with #2 or hopefully ULS being turned on for anonymous users, whichever done earlier.