I think it would be quite messy to have a top level content publication with multiple languages, but to try and answer some of your questions and add some of my own thoughts
1) Translation manager - I'd imagine this would be a little bit of hassle as TM needs to know what the language is and where to localise in a child publication, the configuration and rules around it wouldn't be very nice.
2) Publication targets - I don't think there would be too much issue here.
3) Content Delivery - I can think of a load of scenarios here where querying for content would likely mean querying for the content / language, where normally CD is happy to assume anything in a given publication ID is of the same language.. (well it doesn't put the developers creating it do.... multi-language would mean you need to do a lot of extra work on top
4) CMS Security - how do you manage different teams in the world working on different language contents? This could start international wars!
5) Template logic - checking if something is local/shared etc is pretty much out the window.
.. I think in short, it's a really bad idea, I'm struggling to think of good reasons to have it, are you faced with a client where this would make sense?