Not necessarily - we (the audience hear English -- at least in the U.S. and Canada and England, etc.) - but that's just because the UT on pout TVs broadcast that so we understand. Thy could ACTUALLY be speaking Esperanto for all we actually know<-- and THAT could be "Federation Standard".
Well, a pre-JJ example is TUC. It shows Chang speaking Klingon with it being translated, then the camera pans around and he's speaking English. The message to the viewer is that he's still speaking Klingon (indeed, his line about not waiting for the translation makes no sense if he's actually speaking English), but it's just shown in English for our convenience.
There have been specific references to the language depicted on screen actually being English in-universe, including what Saru said in this episode. Kor
The UT on your TV just doesn't to confuse you... [IE I'm pretty sure when the show is dubbed into German or Japansese - Suru will state "The bridge crew is now speaking >Insert language of dubbed contry here<. Of couse for the countries that can't afford to dub, and just sub-title, who knows. ]
Actually, I don't know about. I wonder if "Federation Standard" is in fact its own thing, borrowing elements from various Federation languages in a phonic array that's almost like a code that's easy for the UT to disassemble and reinterpret and reassemble as needed. Kind of like a programming language is to assembler code and then to binary and them back up again.
Pike identified the distress call in New Eden as Federation Standard (our first canonical reference to that). The distress call was made in 2053 by soldiers in an American church. It might be English. It could be Spanish or French or something. Probably not Esperanto.
Plummer is always speaking the lines that we hear in that scene though, if I recall correctly. Which is the same thing that was happening in the DSC scene.
We just have to use our imagination and pretend something else is happening onscreen. Just like when the Enterprise shows up these days.
Someone asked about Wolof? Turns out that the answer is African. https://en.wikipedia.org/wiki/Wolof_language http://omniglot.com/writing/wolof.htm
Esperanto isn't really popular worldwide. Most people use english for speaking with foreigners. Therefore i really doubt it's Esperanto. IRL there are more people speaking Klingon than Esperanto