Joseph Smith frequently replaced the biblically styled hath with has in his editing for the 1837 edition. In this instance, the 1849 LDS edition replaced the secondary has with hath, probably because of the high expectation of “the Lord God hath” (the biblical phraseology, as in “for the LORD God of Israel hath spoken it ” in Isaiah 21:17 and similarly elsewhere in the King James Bible). This restoration of hath in the 1849 edition was probably unintended since nowhere else did that edition change has to hath. In fact, there are only a few sporadic incidents of has being changed to hath in the editions:
In general, the critical text will restore the inflectional form found in the earliest textual sources. For further discussion, see under infl al endings in volume 3.
Summary: Maintain in Alma 34:8 the hath in “for the Lord God hath spoken it” since this is the earliest reading (the reading in 𝓞); although in this instance Joseph Smith replaced hath with has in his editing for the 1837 edition, hath was restored (but unintentionally, it would seem) in the 1849 edition probably because of the high expectation of the biblical language “the Lord God hath ”.