My team will be paying another company to do language localization--it's noted this is possible to do with XLIFF files: https://helpx.adobe.com/robohelp/using/translation.html. We are interested in 4 languages at the moment, so I was verifying the supported languages via https://helpx.adobe.com/robohelp/kb/foreign-languages-supported-robohelp.html
We are interested in Japanese. It says "We also offer a specific version for Japanese, With Japanese Interface."-- what does that mean? What is the specific version? Does anything special need to be done?
That page, from the looks of it, is very old as it mentions only CHM and Winhelp outputs and Robohelp 9, all of which are at least a decade old (Winhelp hasn't been supported for ages).
What you want is this page for the Robohelp user interface languages: https://helpx.adobe.com/robohelp/using/rh_system_requirements.html
Translation is something completely different. For information about RH2019 New UI and Classic version see: http://www.grainge.org/pages/authoring/translation/translation.htm
For translation using the new translation feature in RH2020, use the link on that page to go to the Robohelp Tour.
You need to differentiate between interface languages and topic content languages.
Japanese has long been supported for the UI. In Classic versions you had to set which language you wanted to use at the time of installation. If you wanted to change it, you had to uninstall and reinstall. In 2019 New UI and in 2020, you change without uninstalling and reinstalling.
The translation feature is new in 2020 and as Amber has said is described in the RoboHelp Tour.