Could the Preservers and the Sky People that Chakotay's people encountered in the long past and in the Delta Quadrant be the same species?
Again, I don't think it makes sense to treat the Preservers as a "species," because no entire species devotes itself exclusively to a single activity. They're probably more of an organization or a movement.
And it's unlikely the Sky Spirits had anything to do with them. According to "Tattoo," it took the Sky Spirits generations to travel from the Delta Quadrant to Earth, and on their last visit, 12 generations before the episode (which would be 3-400 years before if we assume human generations, i.e. the late 20th or early 21st century), they found no survivors of their descendants and assumed they'd been wiped out. So the Sky Spirits missed the whole era when Native American populations were devastated and only arrived in the aftermath.
Most of those random Earth-like planets that have a similar culture to old Earth, I would imagine are using a different language, but the universal translators are working for both the Enterprise's crew and us the audience.
The Roman peoples should be speaking a Latin based language if they are originally from Earth, and the culture influenced by a Federation Historian should be speaking German, if they adapted the Nazi culture entirely.
Sure, that's the usual assumption, but "Bread and Circuses" made it quite explicit in dialogue that the natives were speaking American English.
http://www.chakoteya.net/StarTrek/43.htm
"Complete Earth parallel. The language here is English."
"Colloquial twentieth-century English. An amazing parallel."
This was perhaps done to justify the use of English writing in the magazine, the TV equipment, and the like.