Languages aren't codes for each other

One of my facebook friends whom I don't actually know is a native English speaker living in Japan who was recently (well, recent when I started writing the draft of this post months ago, anyway) vexed by a school worksheet. It had a series of example phrases written in both Japanese and English that included the following:

"Don't use Japanese. Speak English."

"How do you say this in English?"

The teacher instructed the students to cross out "in" in the second example, and when pressed on the issue, explained that since both examples contain 「英語で」, it can't be right for one to have "in English" when the other just has "English". Which, though it makes sense, simply isn't correct.