Split from T334791: Implement interproject magic link like interlanguage magic link in MediaWiki core instead of Wikibase. "In other projects links" is a existing Wikibase feature.
Mockups:
Without language variant:
With language variant proposed in T334688: Vector-2022: Try to improve language variant menu selector visibility :
Note: Alternatively we can do both T334791: Implement interproject magic link like interlanguage magic link in MediaWiki core instead of Wikibase and T334793: [RelatedSites / Vector 2022] Move to page header so that "In other projects links" front-end code in Wikibase would no longer be needed (migrating to core).