The Lubavitcher Rebbe said that although Yiddish originally was spoken to refrain from speaking Lashon Kodesh, since it was used for Torah study and mitzvah observance for over a thousand years it gave it a measure of sanctity beyond other non-Hebrew languages,9 similar to the holiness conferred to a physical object used for a mitzvah.10
9. This is similar to what we find in the Talmud as well as in halachah, that certain languages such as Aramaic and ancient Greek have a certain level of holiness above the average language. See Jerusalem Talmud, Megillah 1:9, and Babylonian Talmud, Megillah 8b.
10. Additionally, by using the language of Yiddish for Torah and spiritual purposes, the rabbis were able to spiritually elevate the language. See Likkutei Sichot, vol. 21, p. 448; Sefer ha-Sichot 5748, vol. 2, Vayeishev, sec. 5.