I believe Sung Si-kyung's English is mostly due to effort

It is known that he lived in Hong Kong for two years starting from age five, but it is difficult to become proficient in English with that much exposure alone.
If he had attended kindergarten locally, he might have developed a sense for conversation, but
if you don't use it for a few years after returning to Korea, that sense disappears.
One must make consistent efforts to maintain that sense and improve vocabulary and sentence structure.