A possible explanation would be that many people may cheat on CrowdFlower's language proficiency tests and pass them even though they do not speak the respective language at all: As mentioned before, the English sentences are presented to the workers as images, so for someone with a sufficient knowledge of both Japanese and English simply translating a sentence themselves should actually be less work than presumably entering it into some automated translation system by hand, or even looking up every single word on its own.
we did a second run of the translation task, which only differed from the first one in that now only workers from Japan were allowed to work on it.