LibreTranslate is a free MIT-licensed engine. Public instances rate-limit — if you hit "too many requests" or a mirror is down, pick another URL from the list or self-host; Theseus falls through to the others automatically. silentmode.st/libre is ours and answers reliably, but only covers English, French, German, Greek, Russian and Spanish. Google (unofficial) uses translate.googleapis.com/translate_a/single — no key, but unofficial and Google can break it at any time.