Well, not anymore they don't. The Yoshi in PMTTYD spoke perfect English (though that may be because he hatched among English-speaking beings). I think one of the Yoshis in M&L:SS read a poster, though they still spoke in parentheses indicating Mario or Luigi was translating. The Yoshis in Paper Mario spoke English... and there's nothing to contradict that. I'm pretty sure the Yoshis in M&L:PiT spoke English, but they might have had the parentheses still. In all the other games...Yoshi just says "Yoshi!" all the time. Oh, in SM64, he spokeEnglish (with one error, probably not intentional anyway). That's all I got.