How do I explain away the fact that the people from the other universe who come to earth are able to speak English?
I mostly write secondary world fantasy that has no ostensible connection to our world or transfer of characters between, so I just assume their language has been translated to modern English sans obvious anachronisms.
I suppose it's different if the novel is taking place in a portal world or a SF setting. If aliens are visiting Earth for the first time, one could assume they've studied the culture and languages for a while and learned some of them first (or built some kind of translator device).
The novels I read where a human goes to another universe or a fantasy land never seem to explain how that human is able to speak the language of that land or how come everyone there speaks English.
Portal fantasies are kind of odd that way. I haven't read any lately, but I'm sure I've read some where the person had to learn the local language or obtain some kind of magical translation. I suppose another option is to imply that there's been enough cultural cross-contamination via portals that the dialect is similar, at least, and not too hard to learn.
Can you think of any ways this has been satisfactorily explained in any novels, especially sci fi or novels set in different universes or fantasy lands?
There is the Babel fish in Adams's work, though that was satirical. I'm re-reading a SF novel right now where the protagonist is an alien, and the "English" the book is written in represents her native language. The other aliens speak their own language at times (which she may or may not understand, depending), or they speak her language or a sort of pigeon (represented in the text) language.
The one human character is an outsider, and he only learns a few words of the MC's language directly. They mostly communicate, brokenly, via a very unreliable translator program they've created, and the human is illiterate in the aliens' language as well. The translator "tapes" are a specific plot element (this book was written in the 80s, and the writer failed to anticipate the obsolescence of tape as a recording media, though in her more recent works the term "tape" becomes metaphoric for any recording device), actually, and the human is something of a pawn in alien power plays.
So SF writers do deal with it. As for fantasy, there aren't that many portal fantasies I've read lately, so I don't remember all the ways they were dealt with. The character requiring time to learn the language first or some kind of magical translation are options (with SF, it could be mechanical translators). Sometimes an ongoing or magical connection between the fantasy worlds is assumed, so they simply speak the same language. Some, especially kid's books or lighthearted fantasies (think Narnia or
The Dragon and the George) simply ignore the issue. Same for light SF stories like
Star Wars. Perhaps a universal translator or some common trade tongue is assumed in that universe.
I think writers make decisions about these kinds of things with an understanding that readers will suspend disbelief when a speculative element is introduced, or something is not explained, if it's required to make the story proceed smoothly.