"Maou" (Janus's Japanese name) means, according to the translators, "Dark King" or "Witch King". "Magus" is a title, not a name.
Strictly speaking, "Magus" is a title rather than a name, true, but in the Middle Ages he went by no other name. There's no problem with his continuing to go by the "name" Magus in an English-language playthrough, since it's the name that the party knows him best by.
You're a bit off on the translation, though. The word "Maou" carries with it some ambiguity depending on how you translate the kanji (see below).
The Fiends (Yes, "Fiend" is far more accurate. Their original name was "Demon") simply gave him that title.
There are a number of reasons (beyond simply the preservation of canon) why the term "Fiend" is inappropriate for the Mystic races. The first being that it's a rather pejorative translation that loses the subtlety embedded in the original Japanese name. The Mystics in Chrono Trigger are called "Mazoku" (魔族). Zoku (族) means "race" (kind of), but "Ma" (魔) does not necessarily mean "demon." It is also used extensively throughout Japanese fantasy and fiction to mean magic, or express in derived words a connection with the supernatural or the occult. (as in Final Fantasy, where the Japanese names for Black and White Magic are "Kuro Mahou" and "Shiro Mahou," both using the 魔 character -- would you translate Black Mage as Black Demon?) In this reading, the name "Mystic" clearly makes the most sense.
Both the name "Fiend" and the reading of "Mazoku" to mean "Demon Race" are unequivocally negative, blackballing the Mystics as inherently evil (despite the fact that the Mystics are redeemed after the party defeats Ozzie's Fort; and later, in Chrono Cross, it's stated that Mystics send scientists to aid in the Chronopolis project). I think that this takes something away from the original Japanese use of "Mazoku"; at least when the race is called "Mystics," the reading can remain neutral, and these creatures can transition easily from being villains to friends with the movement of the story. This is the best we on the English side can do, since we don't use kanji, and our written language doesn't benefit from the kind of nuanced interpretation that Japanese players get.
And also, Schala does indeed call him Janus in Dream's Epilogue, which pretty much seals the deal.
This doesn't really seal any deals... of course she'd call him Janus. That's the name
she knows him by. Atropos calls Robo "Prometheus" -- does that make Robo or R66-Y untrue? No. In the context of the English language translation, both Janus and Magus are names the character uses.
