Kaz is known to speak English, he does it fairly well. However when you're dealing with things that need to be worded extremely delicately and carefully.. speaking in a foreign language probably isn't best.
This way, we have the cross reference of having him saying everything in his own language to avoid any confusion caused by a simple mistake in wording.
True, you just wonder why Kaz doesn't let there be a second "face" to PD or take the time to learn English in more detail. I'm not saying he should or has to of course, it's up to him, it just strikes me as being a bit odd to call a press conference in the UK and giving it via a translator. I don't know how hard translating is but I'm sure Translator San didn't have the time and memory to translate everything as Kaz said originally.