The Wormhole wrote:
T'Pol likely was actually speaking English given UTs weren't as common back in the day.
She specifically says she's speaking English in Broken Bow (something like "I was instructed to speak English") after Hoshi tried to speak Vulcan to her.
I don't think we can extrapolate the extent that everyone speaks English on Earth, but it might be that English is advantageous to learn in Starfleet. I don't think it's ever entirely possible to reconcile the random times Klingon shows up, but I'd suggest that Worf is speaking English rather than being translated. Quark, on the other hand, apparently speaks no English and I suspect that the Bajorans (and Odo) are the same way.
Of course, it's a plot device done for convenience 95% of the time, so I wouldn't think too much about it.