When sending out a MassMessage using "Page to be sent as a message:", target wikis in other languages than English will not receive the message if the page to be sent is in the user namespace (and probably all other namespaces except for the main namespace). This is because the namespace gets translated.

For example:

17:13, 16 November 2021 Johan (WMF) talk contribs sent page User:Johan (WMF)/Android test as message to User:Johan (WMF)/Tech News test list (How should communication work in the Wikipedia Android app?)

would reach English Wikipedia and Commons. But not French, Swedish or Arabic Wikipedia, because the "User:" part would be translated. So in French, we get this error message in the local logs:

18:13, 16 November 2021 Delivery of "How should communication work in the Wikipedia Android app?" to Discussion utilisateur:Johan (WMF) failed with an error code of La page « Utilisateur:Johan (WMF)/Android test/en », du wiki « metawiki », sélectionnée pour être envoyée comme message, n’a pas été trouvée.

"Utilisateur:Johan (WMF)/Android test" doesn't exist on Meta.