RaShI interprets the phrase at the beginning of Parshat Devarim (1:5) "Ho'ihl Moshe Bei'eir Et HaTora HaZot" (Moshe began to explain this Tora), in accordance with the Mishna in Sotah 32a and Midrash Tanchuma Devarim #2, i.e., that Moshe at this point began to present the Tora in "Shivim LaShon" (seventy languages.) It would appear that the concept "seventy languages" is not to be taken necessarily literally, i.e., that the number seventy is exact and that no more nor no less languages and dialects were extent, but rather metaphorically in the sense of invoking every type of verbal expression with which people of at least the surrounding areas were cognizant. This seems to be the sense of R. Yochanan's statement in Menachot 65a, to the effect that since judges comprising the Great Sanhedrin were supposed to be able to hear testimony directly from the litigants and witnesses, rather than via a translator, they would be required "Yodim Shivim Lashon" (to know seventy languages).
Some commentators understand the basis of the contention that Moshe, prior to his death taught and/or reviewed the Tora in seventy languages to be the parallelism between the phrase in question, and a similar phrase at the end of Devarim, describing what is to be done with a set of large stones (see Devarim 27:2 ff. and Yehoshua 4) that are to be erected by the Jews, (27:8) "VeKatavta Al HaAvanim Et Kol Divrai HaTora HaZot Bei'eir Heitev" (And you will write on the stones all of the words of this Tora accompanied by a good explanation). Here too, RaShI notes that seventy languages is suggested by the term "Bei'eir Heitev". Sotah 35b, due to ambiguities in Devarim and Yehoshua, and carrying the parallelism to another level, posits that there were actually three sets of 12 stones: the first was erected in the plains of Moav as part of Moshe's final presentation to the people prior to his death and before the Jordan was crossed, the second placed in the Jordan itself, and a third set located in Gilgal.
Since at least the latter set of stones would serve as a public monument and announcement to all passers by, Jew and non-Jew, regarding the Divine Law of the land to which all inhabitants were expected to conform-while some Tora laws apply specifically to Jews, there are many laws that apply to non-Jews as well, e.g., the seven Noachide commandments and their detailed applications-it is understandable to assume that it would be insufficient for this information to be recorded in a single language known only to Jews. It would appear, in light of the assumption that multiple languages were being presented, that artifacts, comparable to the Rosetta Stone, but far more sophisticated in terms of the number of languages recorded thereon, was being created. However, it is more difficult to account for why the presentation to the Jewish people by Moshe necessitated the utilization of seventy languages. Even taking into consideration the "Eiruv Rav" (the mixed multitude), comprised of many nations, that joined the Jews upon their leaving Egypt, to assume that a multi-lingual presentation by Moshe would be required, after the forty years of wandering in the desert, is difficult to fathom. Two intriguing explanations that draw a distinction between Moshe's presentation to the people referred to at the beginning of Devarim and what was inscribed on the stones mentioned at the end of Devarim, in contrast to the assumption of Sota 35b, are offered by R. Yaakov Tzi Mecklenberg in his commentary HaKetav VeHaKabbala, and R. Avraham Shmuel Sopher in Ketav Sopher.
R. Mecklenberg suggests that the term "Lashon" language, can connote "intentions" or "meanings", thereby literally paralleling the Rabbinic statement, "There are 70 faces/facets to the Tora" (see BaMidbar Rabba 13:15/16.) While for some, as in the case of a foreigner for whom the entire corpus of law is alien, a translation will initially suffice to at least gain a peripheral understanding of what is entailed, ultimately this will prove insufficient and greater depth and detail will have to be offered in order to reach a point of understanding, let alone mastery and internalization. Consequently, "Mishna Tora" becomes not merely a review of what has already been presented, but a more profound explanation, detailed and nuanced, which would hopefully engender long-term commitment to the Tora's dictates, even when Moshe was no longer leading the people and clarifying for them aspects of the Tora that escaped them.
R. Sopher suggests that the invoking of "70 languages" at the beginning of Devarim to provide insight into Moshe's approach to the Tora commandments at this point, is to be viewed more symbolically and homiletically than literally. Clearly having in mind the social and religious tensions of his age (1815-71), the commentator writes that Moshe was attempting to dispel the false assumption that Jewish law was meant to be observed only while the Jews were living in the insular environment of their desert encampments, or even in the land of Israel, when the culture, as represented by language, was singular and consistent; however, once the Jews would find themselves in a society where the culture was not "Jewish", i.e., the language was other than Hebrew or Aramaic, they would no longer be expected to comply with Tora commandments. Consequently, the "seventy languages" were meant to emphasize that in all times, places, and cultural milieus, adhering to the Tora was expected.
Happily, one does not have to choose between either R. Mecklenberg or R. Sopher, both having provided for the ambiguous phrase in question, a "Bei'eir Heiteiv."
Shabbat Shalom.