post SUL finalization the wikimedia.org/wikipedia.org/wikiquote.org/wikibooks.org/etc. front page should have the ability for users to log in and create accounts similar to the way that actual wikis do.
The open question that we need to answer, is if a new users invokes create account from this screen wikipedia.org which wiki will be considered their "home wiki" if that concept even exsists after SUL finalization.
Since language selector in left sidebar does not seem to work when logged out on index.php?title=Special:UserLogin we should probably infer language from browser/OS when user chooses the Create account option.
If home wiki is transparent to the end user, it doesn't really matter which wiki is designated as their home wiki, as long as it's reasonable. Heck we could probably designate Meta or something, or a "www wikipedia" home wiki that doesn't truly exist (kind of like Multilingual Wikisource).
If it actually matters in a substantive way, couldn't we just ask them which language they prefer?
I'd recommend, in order of if the info exists…
Last wiki used (logged in via)
Wiki with most recent edits
Home wiki (is this meaningful to users?)
Wiki where account was created
In the case of new account creation, we could redirect the user back to where they were before logging in, let them choose, and save that choice. Based on the above rules.