calibre does create romaji *provided the interface language is set to Japanese*. And does so using pykakasi which incidentally was spun out of calibre code to do that by Hiroshi Miura who contributed said code to calibre in the first place.
East asian scripts all re-use the same unicode codepoints so it is *not possible* to transliterate them without using a base language. That base language is the user interface language in calibre.
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