One of the toughest challenges of writing about characters moving from our world to another is having them communicate believably to the folks they find in that other world [or time, or country, or level of reality, or junior high]. You can't just stop the plot for months of language-training. Well, that's what Philip Jose Farmer did in A Barnstormer in Oz, and the result wasn't pretty.
Usually fantasy authors ignore this problem, trusting their readers to go along. Deep down most of us believe our native tongue is the natural one, so of course it would also be the language of mermaids, medieval knights, or aliens who look like giant dust clouds.
But there are some cleverer treatments of the problem out there. Douglas Adams, working on the original and still-the-greatest form of The Hitchhiker's Guide to the Galaxy for radio, invented the babelfish; he made evolution solve his language problem, then bankshot that idea into a logical and theological joke. One of my writing-group colleagues has made her heroine's ability to communicate with everyone part of her plot; the girl becomes the new society's translator, which is interesting enough that we stop caring about how she came to have that ability.
My award for canniest use of intrusive narrative voice to meet this challenge goes to E. Nesbit in The Story of the Amulet. In chapter 3, she describes how the magical amulet speaks to the four children we met in Five Children and It:
I cannot tell you what language the voice [of the amulet] used. I only know that everyone present understood it perfectly. If you come to think of it, there must be some language that everyone could understand, if we only knew what it was.Thus, the groundwork is laid for the children's first visit to another world. The next chapter offers Nesbit's longest attempt at explanation--or a semblance of it, in the form of an explanation from an adult who realizes that she is not making sense and is rather cross that the children might found her out.
Now, once for all, I am not going to be bothered to tell you how it was that the girl could understand Anthea and Anthea could understand the girl. You, at any rate, would not understand me, if I tried to explain it, any more than you can understand about time and space being only forms of thought. You may think what you like. Perhaps the children had found out the universal language which everyone can understand, and which wise men so far have not found. You will have noticed long ago that they were singularly lucky children, and they may have had this piece of luck as well as others. Or it may have been that...but why pursue the question further? The fact remains that in all their adventures the muddle-headed inventions which we call foreign languages never bothered them in the least. They could always understand and be understood. If you can explain this, please do. I daresay I could understand your explanation, though you could never understand mine.Fortunately, that difficult moment is over, and the narrator feels no need to relive it in chapter 6:
(I think I must have explained to you before how it was that the children were always able to understand the language of any place they might happen to be in, and to be themselves understood. If not, I have no time to explain it now.)And by chapter 8 the narrator is quite convinced that this is old business, and we readers know we could never convince her otherwise, anymore than we could convince nanny to let us stay up till midnight or set fire to the cat.
I really am not going to explain again how it was that the children could understand other languages than their own so thoroughly, and talk them, too, so that it felt and sounded (to them) just as though they were talking English.