This is a great idea and this is essential for a
full-blown CMS which WP strives to be.
The VPML guy in the begineing of the thread said that WPMLs code base is about 30% of WordPress. Yeah, that might be true, but it is only necessary to be that big because it's not deeply integrated into the core and the cores db-structure. If you could put a language flag on all posts and metadata directly there would be little need for a lot of logic concerning this and it would not hurt performance very much for those that does not use this.