Translate operates at a large-chunk resolution, and one of the insights in solving the problem was the idea that you can often get a pretty-good-enough translation by swapping a whole sentence for another whole sentence. So they ingest vast amounts of pre-translated content (the UN publications are a great source, because they have to be published in the language of every member nation), align it for sentence- and paragraph-match, and feed the translation engine at that level.
It's created an uncanny amount of accuracy in the result, and it's basically fed wholesale by the diligent work of translators who were not asked their consent to feed that beast. Almost nobody bats an eye about this because the value (letting people using different languages communicate with each other) grossly outstrips the opportunity cost of lost human translator work, and even the translators are, in general, in favor of it; they aren't going to be displaced because (a) it doesn't really work in realtime (yet), (b) it can't handle any of the deeper signal (body language, tone, nuance) of face-to-face negotiation, and (c) languages are living things that constantly evolve, and human translators handle novel constructs way better than the machines do (so in high-touch political environments, they matter; the machines have replaced translators in roles like "rewriting instruction manuals" that were always pretty under-served in the first place).