The key word in the verses that pertain directly to the problem (Ether 1:34-37) is confound. What does it mean when the record states that the Lord "did not confound the language of Jared"? Does it mean the same as saying that the Lord did not change the language of Jared? If so, Jared and his people apparently spoke and wrote the language of Adam, because so far as we know there was only one language before the "great tower" of Babel. However, another possible interpretation is that the language of Jared and his brother (and their friends and families) was changed, but it was changed the same for all of them so that they continued to understand each other and thus they were not "confounded."
In his book The Way to Perfection Joseph Fielding Smith indicates he believes Jared and his people did speak and write the Adamic language even after they arrived in the promised land, which was "choice above all the earth." (Ether 1:38.) (The Way to Perfection [Genealogical Society of Utah, 1931], p. 60.)