Hi!
As of now Extension:RelatedSites is used on Wikivoyage only, but I think will be good idea to use this extension for inter-project links unification across other WMF projects.
Currently different templates present these links in different fashion. This introduces interface inconsistency as well as require maintaining and usage of set of templates on various projects.
Extension:RelatedSites need some care:
* it should allow project names localization; * it should be able to extract links from Wikidata.
Tools | Data item should be displayed by this extension for consistency with other projects.
Eugene,