There was an interesting episode of Farscape, where all the main character loss the use of their translation microbes. None of the them spoke any of each others languages.
When Worf is surrounded by other Klingon, I would assume that they are all speaking a Klingon dialect. When Deanna and Laxanna are in private, surely they're speaking a Betazed language. If everyone one on TNG were speaking different languages, and hearing others through their UT's, that would be kind of cool.
What is Beverly Crusher actually speaking most of the time? Gaelic? Certainly in the episode that took place on the Scottish culture colony, the locals were speaking it.
The Native Americans who left Earth to preserve their culture, they were speaking English?
thought that Major Kira wasn't speaking English, not any at all. In the first episode of DS9 my impression is that she was assigned to the station at short notice, and her disdain for the Federation in general certainly wouldn't have lead her to previously learn one (of thousands?) of the Federation member languages. But she spoke English better than I do. That Kira would have learned Cardassian I could see, but English?
Picard: "Le thé, Earl Grey, chaud