One of the first tasks in writing a fantasy novel is world-building. I’ve written several posts on the topic now, but I want to return to a point I have previously made just to emphasize it. Namely, any world you create should be understandable to the reader. That’s basically a no-brainer, and I wouldn’t expect someone to violate that premise, but it is worth discussing. Creativity is great, but there is such a thing as too much creativity.
J. R. R. Tolkien invented the Elvish language for his books set in Middle-Earth. Which is very cool, and very fine. But he didn’t write his books in Elvish, or even the dialogue of the books in Elvish. Sure, there were a few poems in Elvish, but he usually provided an English interpretation. He knew that writing substantial parts of a book in a language only he was conversant in was not a good idea. Hence, the modern writer should feel no shame in inheriting certain aspects of Earth for their fantasy world. Although there is a pull in one direction to make your world as unique and as different from Earth as you can, this pull is not absolute. If it were, the best fantasy books would be complete gibberish.
Some things that should generally be the same as they are on Earth: language—you can call your language something else like Common or Emarin, but on paper it should be written as English (or some other Earth language with a large population base); and the number system—even if your main species has twelve fingers and twelve toes, the number system they use should be base 10. Those are the only two things I can think of (today) that should essentially remain the same as they are on Earth. Everything else is up for grabs, but there should be a word of caution: a reader can only handle so much new material at a time. You must strive to strike the right balance. You can’t overwhelm the reader, else they will lose interest. That means you must keep things similar enough to Earth that you can tell a coherent story. For example, although it is cool to use the occasional unique plant or animal designed just for your world, developing an entire ecosystem is probably going too far. Most of your flora and fauna should be basically Earth-like with only a handful of exceptions. The world itself should probably be a planet or something equally easy to grasp (a hollow world, etc…). You don’t want the reader to work too hard to understand your novel.
All that being said, take my advice with a grain of salt. In the end, it is fantasy literature we are talking about.