Rashi on Deuteronomy 1:5/רש"י על דברים א׳:ה׳

1הואיל means HE BEGAN, just as (Genesis 18:27) "Behold, now I have begun (הואלתי)" (cf. Sifrei Devarim 4:1).

2באר את התורה [MOSES BEGAN] TO EXPLAIN THIS LAW — in the seventy languages of the ancient world did he explain it to them (Midrash Tanchuma, Devarim 2; Genesis Rabbah 49; cf. Sotah 32a and Rashi on Deuteronomy 27:8).