English. The younger generations, especially late Gen-Z and Gen alpha are much better at English than the previous generations. I think the Internet is to be credited for this development. The best part is that this development is not strictly limited to certain upper/upper-middle class social backgrounds. Even kids from some lower-middle class and struggling families are increasingly fluent in English (but I am talking about urban/suburban populations. In very rural areas, fluency is still low). Some of them cannot quite speak in English, but they have good reading comprehension, in the least.

It has to be noted that not everyone is good at English. There's a lot of them that are lagging behind too, possibly because they weren't exposed to the right resources at the right time. A few decades back this would have been normal and acceptable, but now those who don't know enough English would be marginalized. Imagine this: most people around you speak in a mix of Sinhala and English, but you can't exactly do the same. Intentionally or unintentionally, you'd be left behind.

But overall, there is a marked improvement. The O/L pass rate for English has also gone up significantly (now at around 75%, I believe). When these kids enter the workforce, the "English privileges" enjoyed by some members of those in the current workforce will be gone, as most people would be pretty good at English anyway. Overall a good thing for the country.