In general, translators get credited for their translations, since they're creating something. It's a skill to take a text and make it accessible to people who wouldn't otherwise be able to read it. There's also the creative element of writing a translation that captures the style and quality of the original text. In this case, though, "plagiarism" doesn't seem the be the right description. Rather, you used a translation program as a tool to improve your writing. I'd say the teacher has reasonable grounds to insist that you don't rely on technology to give a false impression of your writing skills. It apparently doesn't reflect your actual quality of work if the teacher noticed it change in the first place.

While it might be common sense not to do this, if the teacher didn't make that clear prior to giving the assignment (I'm guessing she didn't because it's a pretty obscure guideline -- who expects the student to do something like this?) it's a bit unreasonable to flunk you rather than give you a chance to re-do it, imho.