I have seen some debate that “Marth” is a mistranslation of マルス (marusu) and that his name should be “Mars.” So I decided to research through old, Pre-Melee sources.
Evidence for Mars
The anime localization. A Nintendo Gameguide article promoting Fire Emblem Fates later claims Mars was a mistranslation here.
The planet/god Mars is spelled マールス or マーズ, with a long ma. マルス is a spelling of Mars the god, but appears to be uncommon. The Roman deity was certainly an inspiration for his name, due to Gradivus, one of Mars’s epithets, becoming the name of a weapon, along with all the other classical inspiration for FE1. Though they clearly have always wanted to differentiate the pantsless prince from the god, picking an uncommon spelling that they may not even have been aware was used to refer to the god sometimes, and a completely different romanization, “Marth”.
Interviews give the impression that Kaga ran a tight ship, and is likely he specified “Marth” as the official name to use for all the merchandise. Non-protagonists ended up with a million different names though. “Mars” has been used occasionally in internal filenames in more recent games, but that’s post-Melee, and is likely just informal shorthand. The only place I’m aware of “Mars” has been officially used is in the Fates character book.
This was intended to be an unbiased investigation, but it’s hard for it to appear so when the results are pretty unanimous.