Why was the translation of the Torah into 70 languages considered a such a calamity, if Moshe explained the Torah to the yidden in 70 languages, before they entered Eretz Yisrael?
You are asking a very nice question, in fact let me add to it. The gemora ( Shabbos 88b) says that each of the Aseres Hadibros were heard in seventy languages? A number of commentaries say that the calamity of the translating the torah into sevety languages was because they had to switch ten different things, in order that Talmai should not make mistakes in understanding the torah. Since they made changes to H-shem’s torah that was considered a calamity, because it wasn’t the pure torah the way it was given.