Why do you think that having a shared top division and a mixed league is better than just having a mixed league? Regardless of the choice, the same problems exist in the form of a potentially different language, culture and a general unwillingness of several users to participate in a league with users from other countries.
It's an inbetween solution, and is based on a real life solution.
Real life Adriatic league has best teams from Serbia, Croatia, Bosnia&Herzegovina, Slovenia, Montenegro and Macedonia, which is pretty strong and competitive.
All countries have their own national league systems, but they are much weaker.
So in BB if you want to be a top dog, you have to make a good team and fight with a quality opposition in a shared Div I. If you have a solid team, you're in your national top league(which is Div II), and if you tank you have to do it in Div III.
It also adds better competition, cuts down on the overall number of Div Is, but preserves national leagues as well(so it's an inbetween solution).
Another pro is that most active teams will get to the shared Div I, and they are more likely to put their differences aside, and enjoy the game instead.
I think it's much better than to mix up everyone.
Also, this way draft could be more easily solved. Lower divs simply get national draftees, and the only problem remains in top div.