Tseng's name has been fucked up since time began. Looking at his entry in the hierarchy, it's written as "Tson". Tsu-o-n. It's clearly legible here. I think the confusion is because they were intending it to sound like a Chinese name and from what I've seen, those tend to get mangled on their way into Japanese. XD Though I'm lost as to how "Tson" is apparently supposed to represent "Zeng".

... Hey, I wouldn't've complained. 8D Or possibly 15 minutes of Zax being a badass, and 5 minutes of OTP time.