I was thinking this myself yesterday so had a little poke about. Using exising code in the fusion package I have made a simple way of doing it.
The maincore file needed some tweaking.
I have modified the most up to date maincore file to make it possible in version 7.
You will need to activate the locale fields in the user fields admin section then it should all work :)
I would mention like i found out you should test all the locales you have installed to make sure they work as i found CATALAN had an error which then wouldnt even let me view the site so i have to change it back via the database (or fix the page ;) ).
Unfortunately this is "just" a locale system, not a full language system (when you can input different data according to the chosen language) like m-Fusion used to be...
([size=9]"just" meaning I'm not a coder and I'd be proud to be able to build it[/size] :))