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.
My first reaction was that "How do you say this English?" isn't even grammatical. That's not quite true, however. If I were to point at some English text and ask that question, the sentence would be valid, though still rather awkward. I'd probably leave off "English" in that question, or ask "How is this pronounced?" instead. Unfortunately, that's not what the accompanying picture shows, or what the Japanese sentence means. The example question is actually asking what the English word for a specific object is. I would translate the Japanese sentence as "What is this called in English?" I also find the original example's English sentence a bit awkward for that usage, though not incorrect.
It's anyone's guess why the first example was deemed authoritative rather than the second one, particularly since "Speak in English" would have been valid. And if I may go on a bit of a linguistic side note, "speak English" and "speak in English" have slightly different nuances. I'd say that the first indicates what to speak, while the second indicates how to speak it. Regardless, both mean functionally the same thing unless you want to split hairs.
Anyway, Japanese has things called particles that function as sort of a metadata for words, indicating their role in the sentence. が (ga), for instance, indicates a subject, while は (wa) indicates a topic (which isn't quite the same thing), and を (wo) indicates a direct object. The で (de) particle in the examples, very roughly speaking, indicates the means by which an action occurs, which is broad enough that in different contexts it can mean anything from the tools used to facilitate it to the location where it takes place. Depending on the specifics, its role in a given sentence can be similar to any of "at", "with", "in", "using", or various other English words.
Getting back to the point, it's frankly silly to expect English to stay consistent with Japanese. English isn't even consistent with English! Take the two sentences "John gave the book to Mary" and "John gave Mary the book". They have exactly the same meaning, and if translated into Japanese, would almost certainly result in identical phrasing. Despite this, and the fact that both are perfectly natural and grammatical, the word "to" appears in one of them, yet is nowhere to be found in the other. Furthermore, adding "to" to the second one would be awkward at best, and removing it from the first one would be outright ungrammatical. Sometimes it feels like English is consistent only in its inconsistency.
Treating a word in one language as mapping directly to a word in another language can have its uses. That's particularly true when it comes to jargon and special terms that ought to be handled consistently. Over-applying the principle, though, mostly just suggests that the translator doesn't properly comprehend the text. Or that they're just being lazy. And, to be fair, I've been guilty of both at times.
Anyway, if you're going for a direct word-for-word translation, isn't it more glaring that the first example changes how many sentences it has from one language to the other? And what about the commas? Throwing around "Speak in English, not using Japanese", though just as unnecessary a change, at least seems like a more obvious starting point. Or we could just go all-out with the literal word-for-word and come up with "Japanese unusing, by English speak." Or not, preferably.