Suspension of disbelief for the reader is contingent on getting the facts right. It doesn’t matter that this may be a fantasy, or a sci fi world. Things have to stack up on the believability level. If I give you a green cat, then I have to feasibly justify why the cat is green. Not that I have, but this is an example.
Setting is everything. If one component is out of place, then the whole thing falls down like a pack of cards. This is where research comes into play. Every single aspect of that which is relevant must be correct. Grounding the fantastical, or the speculative, needs a solid platform of known facts that are correct and that can be correlated as correct.
Does it matter to a reader if they see the word ‘league’? Yep, It is an archaic definition.No, it is not essential to point out that a league is three miles. It is essential to get those leagues into an actual frame of reference that is legitimate. If I find a position A is thirty miles from position B and I want to use the word leagues, then I know it must be ten. Maybe a reader will care and look it up, or maybe they won’t. Doesn’t matter, because I will know. I have to get it right.