In user preferences, I think it would be relevant to add a "know more" link to the opt-out option, because users can ask themselves how do Wikipedia knows what it relevant as a language to suggest, or, basically, what is that feature.
It makes sense to provide a path to learning more about the feature. I'd suggest not to add a separate link, but use part of the existing text:
That seems a clear entry point for those interested in learning more without adding extra text, and it is aligned with other similar cases (e.g., Media Viewer). The link should point to the main information page for the feature, which I guess is should be the same link we use for information of the beta feature (preferably pointing to the relevant language if the information is available in such language).
Since most people seem to be asking about language priorities (as is natural), but they're not getting good answers on the super-messy Flow page, we should make sure that https://www.mediawiki.org/wiki/ULS/FAQ#language-territory is either linked directly or prominently available from the link target.
Since this seemed trivial, I added a patch. Few notes:
- it does not open in a new window, so users might get a warning if they have unsaved changes
- it does not forward the current interface language (because it is not available in the hook), so their mediawiki.org language will be used (almost always English). Then again, there are very few translations of the target page, so maybe this is not a big issue.