Currently, when users create a Wikimedia account they can receive notifications from any project of any language if someone writes on their talk page, via Echo notification for @mentions, and/or via Special:EmailUser — for wikis where they have visited while logged-in.
It's not feasible for a user to go to each project and set their preferences, and most users assumedly don't understand that they can be emailed at all, let alone from different communities. Abusive users can easily evade single project blocks to spam another user.
The default should probably be set to allow communication on wikis where a user has 1+ edits but disallow communication if the user has 0 edits. We should also consider an education feature for users to understand how to control these preferences.
See also: T164542: Epic: ⚡️ General user mute/block feature for features that allow users to control who can communicate with them on wikis where they have logged-in and edited.