Kylo Ren is most definitely multi-lingual. Look at his parents, his uncles, his family droids. He was taught to understand Shyriiwook and Binary alongside Basic. His mother taught him Alderaanian (which is like space Spanish, the man who played Bail Organa is of Puerto Rican decent), his father (and Lando) taught him Huttesse (much to Leia’s dismay), Luke and Han taught him Bocce. As he grew up he wanted to learn more and made C-3PO teach him Rodese, Bothese, Durese, some basic Twi’lek, and Sith. (He made 3PO promise not to tell anyone he was learning Sith, it was supposed to be a dead language and had bad connotations).
Since he learned so many languages at such a young age he still picks up on new languages rather quickly. Upon his posting on the Finalizer he enjoyed getting on Hux’s nerves by speaking a mix of several languages at him. Hux, fluent only in Basic, Bocce, and High Galactic, gets increasingly aggravated as Kylo undermines his authority again and again by giving commands to his officers in their native languages instead of Basic, and he’s too proud to ask for a translation or have a droid there to assist.
So Hux, stubborn as he is, fucking teaches himself as many of the languages he can pick out from Kylo’s words. One day, as Kylo is thinking he’s so clever and insulting Hux and getting at least one of the command officers to agree with him (mostly out of fear of Kylo and not out of malice towards Hux), Hux speaks up in whatever language they’re speaking with a simple “That is enough, Ren.”
I imagine they eventually pull a Boondock Saints and talk to each other around other people in constantly varying languages, confusing the shit out of everyone and acting like proud little shits the entire time. It’s a step towards their future of working and ruling the galaxy together.