This is annoying intermittent bug, but it happens frequently enough to report.
Most of the time I use Wikifunctions with the Hebrew user interface. I see the "Wikifunctions is a new project by the Wikimedia Foundation" Sitenotice banner in various languages: Bengali, Sorani Kurdish (ckb), English, etc. This happens even though the banner has been translated into Hebrew since April.
Here's an example, in which the UI is in Hebrew, but the banner is in Sorani (ckb):
This screenshot is from mobile, but it happens on desktop, too.
This may be related to T359772.