There are plenty of Earth languages he's unfamiliar with. Unlike Uhura he doesn't have a seemingly endless number of languages he can hold onto at a given time and with the universal translators it's easy enough to stick with English and set his focuses on other species most used languages rather than getting drug through the weeds of every planet's multitude of dialects and languages.

Plus, if Felix if from a colony originally there's no telling what he was taught so far as language goes growing up. It could have been a mishmash of several different species and regions all coming together to form it's own pidgin language--no he'll settle for listening carefully and taking contextual clues on any phrases the translator won't really know what to do with.

"Understanding human social customs isn't...always easy. I like that she doesn't leave me guessing."