In order to understand what Korah's and Dathan's issue was you would need to look at Jewish commentaries. Their objection was not because of langauge because of an internal issue since Qorah came from a family of Levites, Qohoth ben-Lewi, that had previously been the leaders before Mosheh and Aharon. Dathan, Aviram, and On all came from the tribe Reuven who were the first born of Ya'aqov (Jaob).
They were not concerned with not needing interpreters. Mosheh was not an "interpreter" he was a judge and he transmitted "exactly" what Hashem to him to transmit. Their specific statement (רב-לכם--כי כל-העדה כלם קדשים, ובתוכם יהוה; ומדוע תתנשאו, על-קהל יהוה) against Mosheh and Aharon denotes this. I.e. "You and Aharon put yourselves in this position of being the head Judges of the people and also the priesthood. It didn't come from Hashem - you made yourself the king and your brother the Kohen HaGadol. You invented that for yourselves. We come from the families that are supposed to be the leaders here. Everyone here is at the same level."
Yet, if one goes to Shemoth (Exodus) when the Ten Sayings were made the people moved further away from Mount Sinai out of being overwhelmed by the presense of Hashem and Mosheh stated that they could all come and hear Hashem closer like him. They stated that Mosheh should be the one to go and talk to Hashem and what he transmitted from Hashem they would do. I.e. Mosheh didn't in any way put himself in that position and the opportunity was already there. So, essentially Korah and his group were putting themselves in a very dangerous situation. Yet, the claim of Korah and his group didn't just come out of nowhere. There were events prior, not mentioned in the written in the text, that detail more of thier interests.
What is interesting is that the Hebrew Torah explains that even Qorah's sons backed away from him and his claimed. Further, On ben-Phelet also did not perish. According to Jewish sources it is because his wife saved his life. Before the big challenge was supposed to happen she confronted him about how foolish Qorah's claims were and that Qorah was looking to be the leader and if Qorah was the leader then that means On ben-Phelet would not be. When he wouldn't listen you took off her head covering publically and went outside. Because she delayed him he did not share the same fate as the others.
The Hebrew Torah was writte in a cliff notes format and the inforamtion between the lines was tansmitted from ancient Jewish communities to their descendants. Not every translator took those Jewish sources in account and some had an agenda to not include Jewish sources.