Shalom Aleichem Rabbi,
iggeret hagra is important for us Litvishe Mitnagdim.
The original is – I suppose – in Yiddish.
Can you find out if the Yiddish ~original text of Iggeret HaGra is available (maybe even for purchase) somewhere?
A shlema healthy week.
I looked around a bit. I did find in one sefer (Pirush Hagra on Shir Hashirim ( R’ Shlomo brevda zt”l) 4 pg. 28) that says that there are those who say that the original was in Yiddish, but that R Zundel Of Salant had a hand written copy of the letter, that he himself wrote, and it was in Lashon Hakodesh. If there is such a thing It would be interesting to see.