The joint
Übermensch is a directional compound: a human over the human, the figure Zarathustra is talking toward. Über is over, across, above. Mensch is a person. Early English said “superman,” which puts the figure in the register of the superlative and, after 1938, of the cape. Kaufmann’s “overman” is a calque that refuses that register. It still says “man.”
French surhomme calques the direction with sur and takes the gendered noun. Spanish superhombre copies the risky prefix. Japanese 超人 is the interesting middle: 人 really is a person, not a man, and 超 really is “beyond,” and the compound is also what you would call a superhuman hero in a different kind of story. The register is available in both directions. Context has to shut one of them.
None of these breaks stops a reader understanding the chapter. They change the costume the figure arrives in. Costume is register. Nietzsche is not helped by a cape.
Why this break matters to a reader
A reader who meets “superman” has been handed a champion. A reader who meets “overman” has been handed a piece of translationese, which at least does not cheer. A reader of 超人 can go either way, and the book has to steer. The philosophical claim is about a kind of human, not about powers. The word that sounds like powers has already argued, wrongly, before the paragraph begins.