We suspect that this is a bug with the on-wiki community-managed wikitext as written by @Ameisenigel, and not something we can properly fix in the MediaWiki software ourselves. Perhaps the Language and Product Localisation team might have some views, but for now I'll mark this as External from our end.

No immediate suggestions from our side. Translated sitenotices (via the sitenotice message) aren't officially supported I think. There is a way to make it behave like a regular interface message, but then it would be translated in a different way.

Hi all, translations exist which is correct, but i observe that the user interface does not call the translated version but always fix EN version on the different Wikifunctions pages (see corresponding discussion. So I think the problem is bound with the selection of UI preferences. More of that i have signalled too that the Wikifunctions main page in FR calls too other page for News which is also in EN although the translated versions of news exists (see discussion).

Direct call to {{Template:Sitenotice/Wikifunctions is a new project}} shows the english version, but {{Template:Sitenotice/Wikifunctions is a new project/{{int:lang}}}} shows the correct translated message.

After making several tests I deduced that calling -> {{MediaWiki:Sitenotice}} provides the right translation results
but calling -> {{Sitenotice/Wikifunctions is a new project}} shows me the EN version. May be investigation should be on where the call to Sitenotice is done for Wikifunctions..