Actually, I am not that familiar with the Holland Big Band. I have seen them twice now, at the trumpet parties of 2004 and 2005. I know the bass trombone player, he is one of two guys at the repair place/shop I go to.
I am sure they all speak English to a degree. Most Dutch people (think they) do. The person singing here does quite well, and in my experience most Dutch enough know English to learn the songs. They don't need to learn things phonetically.
That being said, I usually find that when Dutch people sing in English, the accent bothers me a bit. This is especially so of course with non-professionals.
On the other hand, while English is my native language, living and growing up in the Netherlands for 34 years with my only source of "proper" American English being my mother, television and once-a-year visits to Michigan doesn't help. After about two weeks in Michigan things do pick up quite a bit though, but I am sure you can still tell that I spend most of my time "somewhere else". Same thing goes in reverse too: the Dutch think I sound like J.R. Ewing when I speak English (as they do with most Americans. Yes, really) and say I have an English accent when speaking Dutch. More so when I am tired or very angry. In the land of the blind....
Not sure you wanted to know all that though ...
Regards, Christine