Technically some dialect of Dutch is his first language, but not in general conversation, only among those in a small radius from where he lived. He said Dutch people have to speak English to understand each other if they are out of their own little area. This is going to sound unbelievable but he said they have hundreds of dialects in Holland and you don't have to travel very far before you can't understand what the residents are saying. That is why he was so astonished, and marveled exceedingly, at our ability to travel a thousand miles and never have any difficulty communicating with other Americans.
If you've studied the Low Countries, how difficult it has been to get the people to agree on many issues, and how staunchly set they are in many ways, it isn't difficult to see how this situation developed.
India and the Philippines seem similar to the Netherlands in that English is the unifying, working language of the nation as a whole, although there are a bunch of local dialects or languages. South Africa could also be the same.
Language mutates as Dutch in Amsterdam, then transitioning to Flemish as you head south toward Antwerp, then to Walloon (more Frenchlike) as you continue down to Brussels, then gradually transmogrifying into French as you approach Paris. And up in northern Holland, they speak Friesian (which is the most closely related to English)
That explains a lot. I marvelled at how almost everyone, except for some elderly folk, spoke English in the Netherlands, and fairly American sounding English at that. I didn't understand about the dialects. I suppose all Dutch dialects are derived from German, though I wouldn't say that to a Dutchman to his face.
Two hundred years ago I'd think there were bigger variations in dialect, but with the way Americans move around, growing up in one state, going to college in another, then moving all over as jobs require, this has produced a pretty thorough homogenization of accent. The main exceptions are places like the black ghettos where people tend to stay in the same place for generations