Twelve languages
Machine translation stopped being the problem years ago. What it still gets wrong is how formal to be — a decision English barely makes you take, and most other languages will not let you avoid.
Write “could you send that over when you get a chance” and translate it into German, and something has to be decided that you never decided: du or Sie. There is no neutral option. The same choice exists in French, in Spanish, in Japanese — where it is not one choice but a whole system — and in most of the world's languages.
A translator picks one and does not tell you. Usually it picks the formal one, because formal is the safer default when nobody has said. So your friendly note to a colleague you have known for three years arrives sounding like a letter from their bank.
It is not an error. Every word is right. It is just addressed to a stranger, and the person reading it notices that before they notice anything else you said.
Two passes, ten seconds. Translate first, then run the result through the paraphraser in Casual or Formal depending on who is reading it.
That second pass is the whole reason for having both in one place. A translator on its own cannot do it — it has no view on your relationship with the recipient, and it was never asked.
Use Simple instead when the reader is working in their second language. Shorter sentences and commoner words survive translation into a third language far better, if it gets forwarded.
This is the larger use and the one nobody writes about. Not translation — you already write in English, and you write it well. What costs you is the residue: an article missing here, a preposition that is almost right, a phrasing that is correct and not what anybody says.
None of it affects whether you are understood. All of it affects how carefully you are read, and that is a tax on the argument rather than on the writing. Fluency mode is built for exactly this: it fixes the residue and leaves your sentences your sentences.
It is also the reason institution-wide access is worth more than it looks. The students who lose marks for grammar rather than for reasoning are overwhelmingly the ones writing in a second language.
Spanish is on the free plan. The other eleven are on Pro and above — along with the paraphraser and the grammar check, which is where most of the value in this page actually sits.
Idiom and jokes. Both depend on something the words do not contain, and every translation system — and most human translators — will hand you back something technically accurate and completely flat.
Anything legally binding in the target language should be read by a person who speaks it. A translated contract is a draft, not a contract.