Rabbi Avi Grossman writes:
Concerning the post on English, see Igroth Moshe, OH 5:10, in which R’ Moshe references Megilla 9b, which explains why, of all languages, the Torah may also be written in Greek: Noah blessed Japheth that his beauty would dwell in Shem’s tents, and Javan (Greece) was Japheth’s fourth son, and his language was the finest. It so happens that Ashkenaz is mentioned as the firstborn of Gomer, Japheth’s firstborn, and therefore it was fitting that this blessing, that the Torah be transmitted in Japheth’s language, should also apply to the Ashkenazic, i.e., what became Yiddish. Indeed, as he points out there, the language of the major Yeshivot in Israel and the western world was Yiddish. (I will leave aside the issue that it seems that the original Ashkenaz is not the forerunner of Germany, but rather somewhere in western Asia — see my post about that.) If R’ Moshe is correct, then English, which is a descendant language of German, should also enjoy a partial blessing, and thus also be fit for the transmission of Torah. Indeed, English is now the language of instruction in THE largest yeshivas in America and in some of the largest in Israel.
Also, see Igroth Moshe YD 4:38 for the nuance of R’ Moshe’s approach.