(swapping tags, IIUC this tag might be more fitting here as there might be work needed on the translatewiki side of things)

As shown in T412073: Remove dependency of WikimediaMessages from SearchVue, files in https://phabricator.wikimedia.org/diffusion/EWME/browse/master/i18n/wikimediaprojectnames/ contains localized names for Wikimedia projects in different languages. This is potentially useful for 3rd party wikis (if they are using somthing from Wikidata), for example showing "article in XX Wikipedia". Probably we need to find a better place for such information.

Alternative we can have a more systematic way to generate such information. Though special Wikimedia projects still needs such a message.

for example showing "article in XX Wikipedia".

They cannot be used in a sentence context due to localization issues. Also similar labels are available in Wikidata itself, e.g. https://www.wikidata.org/wiki/Q328

WikimediaEvents has 20 or so messages so might not be worth it

It sounds a bit confusing to move things from an existing wikimedia-specific extension, that're already specific to that extension, to a new extension? What's the benefit here?

Does this mean you want to move all messages from WikimediaMessages and WikimediaEvents? I don't really understand why, either. It would make sense to do it for any messages that will be used by WikimediaCustomizations code, but otherwise, they seem fine in their current places.