There are still some i18n messages in the #page-previews codebase that now should go to the #Cite extension because it's responsible for the #reference_previews .
They are mostly prefixed with `popups-refpreview-`. Probably it's best to:
[x] Look for some guideline how to do that, or ask Raymond what's best
[] Copy them over and rename them ( possibly including the translations )
[] Copy any on-wiki message overrides and make it clear that the old keys are deprecated.
[] Start using the new message keys in the code
[] Wait for the complete deployment of both
[] Remove the old keys from Popups codebase
[] Remove the fallback mechanism for the `popups-` keys in Cite
Note, that `popups-refpreview-user-preference-label` is part of the user settings that are still created by the Popups extension. See {T363162}