"One says that French is beautiful."
Translation:Oni diras, ke la franca estas bela.
There's also the grammatical answer that "oni" is a pronoun signifying what, in English, is called the "generic you," or the older version in which we said "one," as in "One might say..." and "unu" is simply the word for the cardinal number "one," as in "one, two, three..."
It has something to do with ke, yes. For the purposes of determining case, everything after ke will act like a separate sentence, and will ignore everything before ke. If you are considering only "la franca estas bela", there is nothing there that causes anything to be accusative, so nothing in that phrase gets -n. It is true that "ke la franca estas bela" is a direct object for "diras", but it would be awkward to put -n on every word in that phrase, so we do not do that.