It is possible that the 1837 typesetter accidentally dropped the long clause “and because that none other people knoweth our language” since nearly all of it is identical to the preceding that- clause. But another possibility is that Joseph Smith intentionally deleted this clause because it was repetitious. In any event, the 1908 RLDS edition and the 1981 LDS edition restored the deleted clause, probably under the assumption that the deletion was accidental (since it was not marked in 𝓟 by Joseph Smith). For an example where a repetitious clause was consciously deleted by Joseph, see under 1 Nephi 14:1–2.
Summary: Accept in Mormon 9:34 the longer phraseology, although repetitious, since it represents the earliest text; similar repetition can be found elsewhere in the original text.