Shalom! Is it permissible to recite kaddish, and other parts of the tefillot that are in aramaic, in Hebrew? Many thanks.
The nusach of kaddish was enacted specifically in Aramaic, and it is best to recite it as it was enacted (see Rashba 5:54; Tosafos, Berachos 3a; Tur, Orach Chaim 56 citing from Avudraham).
Bedieved, it is permitted to recite it in Hebrew, for Hebrew is of course not worse than other languages — and is in fact much better (for other languages, see Mishnah Berurah 55:1). However, the preference remains to recite it in Aramaic.
The same will apply, in principle, to other parts of the siddur, though there are not many that were originally enacted in Aramaic.