As signatories of the Outer Space Treaty they should give a heads up to the international community, and are liable for any damage that's caused by this event even if they provide warning (but could probably get out of it with enough political double talk).

What's interesting to me is that China haven't officially admitted to losing control of the station, they have only admitting to lost telemetry- so we must rely on external sensors to track it. I suspect (but don't have any real information on why) that they are doing this to avoid some sort of admission of guilt in the event that it ends up hitting a populated area. I'm not entirely sure but admitting you lost control of an orbiting school bus might be seen similarly to a kid that accidentally breaks your window with a baseball- it wasn't intentional, but that kid is still paying for the window. If your window gets hit by something, but the kid never admits that it was his ball ("gee mister, I lost my ball yesterday but I'm pretty sure it didn't go in your backyard"), then maybe he can get away without paying for it.