Oomis, when Rashi needed a word that could not be found in Ivrit, he wrote in French. If the foremost commentary on the Torah is written even partially in French, I think that trumps Yiddish as the Jewish language.
Oh, and before I forget, to the person who said your children use only their birthdays according to the Jewish calendar, I have a similar problem. Since I started putting 5771 on my checks, the bank has been bouncing them saying they are post-dated 3,761 years.