SSH speaks decent english. During the Summer Scent's interview in Taiwan with international reporters (probably the same one that you mentioned), he was rather shy and talked a bit small in volume. But overall, his English is understandable, especially if you'd consider his residency in non-english speaking Korea.
Originally, Seung Heon was a prospective star for Tsui Hark's Seven Swords. Charlie Yeung Choi-Lei, when asked if she was concerned about communication problems with her Korean co-star, Yeung remarked: "I've met with him already. He speaks really good English so I don't have to brush up on my Korean."
(see:
http://www.hkentreview.com/2004/jul...0701to0703.html)
I don't know about his Mandarin and Vietnamese. In the Summer Scent's interview, he said "how are you", "my name is Song ChengHsien" in Mandarin, then he said "hello" and "thank you" in Vietnamese; and that was all.
It's funny though, that he said "Chao anh" (hello, brother) to a Vietnamese female reporter. When corrected, he rectified "Chao anh. chao co" (hello, brother. hello, miss) to her.
SSH carried out his chinese dialogues for his part in So Close while filming. But It wasn't his voice that you heard in So Close at the end. They dubbed his voice!
It'd be interesting to hear him more in Chinese though.