By which I mean fluent enough in more than one or two languages to contribute to the relevant wikipedia and to need to see what's going on in more than one wikipedia at a time?
It seems like a great suggestion to be able to access a global 'recent changes' listing etc, but really, how much use is it to the average user? Somebody who is monolingual, or poorly bilingual is just going to be overwhelmed by surplus information and put off the whole project idea if it seems like you HAVE to be a polyglot to get any sense out of it. If you speak Japanese and Malay it's not going to be a lot of use to you to see a listing full of French and English edits, or vice versa.
You'd be better off for the portal page to have access to each language seperately than to try to jumble them together. Select your language from a dropdown menu and off you go. Want recent changes in Swahili? Choose Swahili from the language box and you'll see it. Want to check up on the Japanese next? Switch over... etc.
I support a global login. I don't see why you should have to have seperate logins set up for the meta and the standard english pedia AND any other language that you may be fluent enough to be interested in. If you want the wikipedias to be one project and not 500 then one login ought to work for the whole kit and caboodle.