Rav Aharon Lichtenstein was my classmate at YU studying under Rav Y D Soloveitchik, who gave his shiur in Yiddish. That may be why Rav AHaron gave his shiurim in Yidish too. After I had left YU I foud out that the Rav stopped using Yiddish because the younger generation no longer understood it. I am not sure if he switched to English or Hebrew [perhaps someone can enlighten us from his experience at Yu?]
When I was in the Rabbinate I spoke twice a year in Yiddish [Shabbos HaGadol and Shabbos Teshuivah] just to keep “in touch” – to the delight of several old-timers in my shul.
The seforim I wrote are all in English – and b’h sold out.
