Is there a significant divide between international and Singaporean students?
At first, I definitely think there is a cultural gap between Singaporean and non-Singaporean students, even more so for students that are not from East/Southeast Asia. There are a lot of specifics about Singaporean life that local students may take for granted. Also, it will take some time before you can understand Singlish (Singaporean English). Adjustments like these always have to be made when you move somewhere.
If it helps, I think Yale-NUS is much easier to get used to for internationals than other places in Singapore, because the community is so much more diverse in general than NUS or NTU. Moreover, my experience has been that Singaporean students are very, very patient and helpful about whatever gaps in your knowledge you may have. Local students are quite eager to introduce others to local cuisine, cultural practices, and common slang. They are even willing to show you around town: both popular and lesser-known spots for food, shopping, or just cool places that they like to visit. My Singaporean friends have also been nice enough to invite me over to their homes for dinner and the like.
Of course, some students feel more comfortable hanging out with people of similar backgrounds, especially at first. But I personally don’t feel like things of that nature remain a significant barrier. At first, it might seem like you are quite different from Singaporeans, but within a few weeks I guarantee that the national boundaries will become less important and you will basically spend time with people that share your values or interests (or that you get along with in general) rather than thinking about it on a Singaporean/non-Singaporean level. Things like your DF group, classes, and whatever extracurriculars you are in are all great opportunities to meet people, the latter especially for people who might be passionate about the same things as you are.
In fact, the only (national) factor of some significance I have noticed is an actual linguistic gap. Particularly for second or third-language English speakers, it can be quite tiring at first to talk to so many people in English (and all the time). So in this respect, it is nice to have someone with whom you can converse with ease. Depending on what you speak, there might not be a lot of freshmen who can converse with you, so perhaps you can make friends with some upperclassmen. But in general, I think a pretty wide range of languages is represented at YNC.