After the success with the mongoose language, Taako, Lup, and Barry were given the unofficial role as crew linguists and translators. Every time the group encountered a world that spoke a different language - which was often enough - the three of them spent the first several months together figuring out the new puzzle. They were impressively successful, even when they had to improvise; one world communicated through clicks and noises that they couldn’t replicate with their voices, and they had to develop a system of claps, finger snaps, and whistles to make comparable sounds. One language was entirely whistles.
The result of these efforts is that Barry, Lup, and Taako are multilingual; they are also the only people on Faerun who can speak these languages fluently.
Kravitz is sitting in the kitchen, watching him cook while doing some paperwork at home instead of at the office. Taako is singing a simple song under his breath without thinking about it, and Kravitz is curious because it’s not familiar. He listens more closely and realizes he doesn’t know any of the words. He doesn’t even know the language.
When Taako and Lup want to talk about something (or someone) without being overheard, they pick any of a selection of different languages and do nothing else to hide what they’re talking about. Barry promises Kravitz that they’re never talking about him; eventually he also starts whispering translations to Kravitz, and Kravitz has to struggle not to laugh.
The whistle language proves useful for reaper business when the three of them have to communicate covertly from a distance. To the untrained ear, they just sound like birds; after they start using the whistles, the group becomes incredibly difficult to ambush and incredibly talented at ambushing in equal measure. (Roswell picks it up, and everyone agrees that they have the best accent.)
Eventually, Taako arranges classes to teach some of the languages at his and Ren’s school. But only some of them - some languages are just for the family.
Taako promises to teach Kravitz because he shouldn’t be left out of conversations; it’s a slow process, but they have so much time. Taako starts with a language made entirely of hand gestures. Sometimes when the morning is too quiet to break the silence or Taako is too lost in his head to say much, he presses a single gesture into Kravitz’ hand.