These arguments are typified in the anti-Mormon book, Everything You Ever Wanted to Know about Mormonism by "Dr." John Ankerberg and "Dr. Dr." John Weldon (neither one of which appears to have a legitimate Ph.D.):
"Mormonism has never explained how godly Jews [sic] of A.D. 400 allegedly knew Egyptian, nor why they would have written their sacred records entirely in the language of their pagan, idolatrous enemies" (p. 284). "How likely is it that the allegedly Jewish [sic] Nephites would have used the Egyptian language to write their sacred scriptures? Their strong antipathy to the Egyptians and their culture makes this difficult to accept. When modern Jews copy their scripture, they use Hebrew. They do not use Egyptian or Arabic, the language of their historic enemies" (pp. 294-95). "[N]o such language [as reformed Egyptian] exists and Egyptologists declare this unequivocally" (p. 294).
Today we know that there was a lot of healthy exchange between ancient Jews and Egypt. Jewish communities existed in Egypt, even a Jewish temple was built, and Jewish people in Egypt in Thebes about 2000 years ago may have even been part of the unfolding Book of Abraham story.
As for the common charges against "reformed Egyptian" in the passage cited above, Ankerberg and Weldon are wrong on several counts--grossly wrong, as shown by Daniel C. Peterson in a noteworthy book review in Review of Books on the Book of Mormon, Vol. 5, 1993, pp. 43-45 (available online). Several modified or "reformed" Egyptian scripts are well known, including forms called Demotic and Hieratic. "Reformed Egyptian" is clearly an appropriate generic term for those writing systems. However, the "Reformed Egyptian" used by the Nephites is described as a language system unique to them (Mormon 9:32-34), having evolved with their culture over a 1,000-year period. It was apparently used for sacred writings, and should have been almost wholly lost with the destruction of Nephite civilization. How can we expect Egyptologists, with typically no training in Central American matters, to know whether such a language ever existed there? Daniel Paterson gives further analysis (Peterson, pp. 44-45):
[W]ho says that the Nephites wrote in Egyptian? That is certainly one possibility, but several scholars (e.g., Sidney Sperry, John Sorenson, and John Tvedtnes) suggest, rather, that the language of the Nephites was Hebrew, written in Egyptian characters. The practice of representing one language in a script commonly associated with another language is very common. Yiddish, for instance, which is basically a form of German, is routinely written in Hebrew characters. Swahili can be written in either Roman or Arabic scripts. Judeo-Arabic, as written for instance by Moses Maimonides, was medieval Hebrew written with Arabic letters. In fact, almost any textbook of colloquial Arabic or Chinese or Japanese aimed at Western learners will use the Latin alphabet to represent those languages. Language and script are essentially independent. Turkish, which used to be written in a modified Arabic script, has been written in Latin letters in the Republic of Turkey since the 1920s. However, in the areas of the old Soviet Union, it is now usually written in Cyrillic (Russian) characters. Likewise, perhaps the major difference between Hindi and Urdu may be the mere fact that the former uses a Devanagari writing system, while the latter uses a modified Arabo-Persian script. So this phenomenon of changing the script with which one writes a language is by no means unusual.
But we need not speak only in theoretical terms. We have, in fact, an ancient illustration that comes remarkably close to the Book of Mormon itself. Papyrus Amherst 63, a text from the second century B.C., seems to offer something very much like "reformed Egyptian." It is a papyrus scroll that contains Aramaic texts written in a demotic Egyptian script. (Aramaic is a language closely related to Hebrew. of the Old Testament book of Daniel is written in Aramaic, and it was the spoken language of Jesus and his apostles. Incidentally, however, a Christian form of the language, Syriac, came to use an alphabet related to Arabic--again illustrating the independence of script and tongue.) Interestingly, one of the items found on Papyrus Amherst 63 is a version of Psalm 20:2-6. Ankerberg and Weldon wonder why "godly Jews [sic] . . . would have written their sacred records entirely in the language of their pagan, idolatrous enemies." Perhaps they should ask them some day, for godly Jews most certainly did (see "Language and Script in the Book of Mormon," Insights: An Ancient Window, March 1992, p. 2).
By the way, Peterson gives a footnote on Ankerberg's claim about Jews exclusively using Hebrew:
The statement "When modern Jews copy their scripture, they use Hebrew. They do not use Egyptian or Arabic, the language of their historic enemies" is quite an astonishing display of ignorance. Since the Egyptian language has been dead for centuries, it is hardly remarkable that modern Jews do not read the Bible in Egyptian. On the other hand, "the first and most important rendering [of the Old Testament] from Hebrew [into Arabic] was made by Sa'adya the Ga'on, a learned Jew who was head of the rabbinic school at Sura in Babylon (died 942)" (George A. Buttrick, ed., The Interpreter's Dictionary of the Bible [hereafter IDB], 4 vols. and supplement [Nashville: Abingdon, 1962-1976], 4:758b). Thus, Jews have indeed translated the Bible into "Arabic, the language of their historic enemies." They also have translated it into the language of their "historic enemies" the Greeks (IDB 4:750b on the Septuagint) and Aramaeans (IDB 1:185-93; 4:749-50, on the Aramaic Targums).
More information and relevant examples are given in the article, "Jewish and Other Semitic Texts Written in Egyptian Characters" by John A. Tvedtnes and Stephen D. Ricks, Journal of Book of Mormon Studies, Vol. 5, No. 2, 1996, and also the excellent FARMS article "Reformed Egyptian" by William Hamblin. And for fun, be sure to see the site, Ancient Scripts--a marvelous collection of information on scripts of the ancient world.