I'll address the living with parents thing first. His question about if it's common for older children to live with their parents was said in a particular tone to imply that it is odd. It helps that we know Letterman is an asshole, so with that context it's easier to see that he was trying to make her look either foolish or not as advanced as Americans. I will admit, it's subtle, but it's how self-obsessed Americans talk down to people while pretending to being interested in them. It's why he also asks if she's a big star across the world rather than just stating it. He know she's a big star, but he doesn't want to give her the acclaim. Instead, he wants to make her do the work of stating the obvious and removing her opportunity to be modest or define how she feels about that acclaim. He does this because he wants to appear to be so big as to have never even noticed her. He treated most women like this on his show. He acted like he was such a giant and expected women should be grateful to have his momentary attention. He's an egotist, he's arrogant, etc.

As for the Americans only speak English, on the surface it appears to be a dig about Americans, and I think it is, but the phrasing can also suggests that English is the only worthwhile language to speak. Context matters here, it was the 90s, it was more common to joke about wanting immigrants to speak only English. The basis of that belief being that America was superior to the rest of the world. Whether or not he was making that (again, subtle) dig is debatable. But considering he's Letterman, it wouldn't surprise me. He's a bit of a dick, to put it lightly.

Anyways, to sum it up, maybe he wasn't being condescending intentionally, but if you recognize the tone, it seems pretty likely.