Actually, I just remembered that Dragon City seems to be set up with an english-speaking minority in mind - all the gravestones are bilingual, even for people like Li qian's grandmother, and Zhao Yunlan has a business card that's almost completely in English (and has his English name of "Mark Stewart"). Based on that, I'd guess there's a suitable pool of volunteers for translation, and we can assume that the populace is better than average at english. Zhao Yunlan is probably at least moderately fluent, but going to offload babysitting duties to other people. :P

I come from an ESL nation, and there is a wide, wide gulf between perfect ESL English we see in fandom and the clunky English with which people can still communicate, but even at the clunky end: people can still communicate. Peter might have to speak slowly and use simple English, but clunky English can still get the point across.

Sang Zan is actually the SID member least likely to speak any english! He doesn't even speak mandarin yet, so his language study attempts would obviously go that way.
- However, the show has zero language barrier during the time travel section. IDK. The Hallows can translate?