I would like to translate in rōmaji but I don't know the code...
To Japanese in Romaji? It's not possible at the moment. To whom will it be useful?
The same motivations for hif-latn. I think that would be useful to the people who only speaks japanese and can't read kanas, kanjis, kokukujis.
But Latin is the main script for Fiji Hindi (at least in the Wikimedia world; I'm not sure about books or other websites).
But Japanese is not normally written in Latin, except in textbooks. The MediaWiki UI is usually not used as a textbook.
I read it and it's interesting, but either I missed something or it doesn't answer my question. Many languages have standardized ways for Latin transliteration, and they are indeed used in textbooks, train stations signs, and some other places where it's useful for people who don't know the language. But is the user interface of a website such a place? I don't think so. Lots of people learn Russian, Chinese, and Korean, but there is no MediaWiki localization in ru-latn, zh-latn (Pinyin), or ko-latn.