Actually,lots of overseas people did have lots of mistakes on wt they said.
But i also think that they have spoken fluent English..
As nobody are gonna care abt the grammar in speaking.
Know what you are talking about and trying to make sense out of it. Also be precise and simple. Just get to the point.
Speak clearly, foreigners doesn't speak fast(for me) at all, just let them understand what your talking about clearly and you'll be alright.