This is a tricky one. It is not enough to just add localized description on AppStoreConnect. Apple shows Languages in this section according to the files localized in your app in Xcode.
Facebook example
Easy way to do this is to change "Localization native development region" field in Info.plist file:
Changing development region
IMPORTANT NOTICE: However, this solution is enough only if you don't localize any other files (solution for this is bellow, you must do some extra steps).
You need to open Xcode, select your project in files structure, select Project (not targets) and add the languages you want to be shown there (in your case Japanese). On the following image there are a couple of languages (don't check the "Use base internationalization" checkmark):
Localizations languages
IMPORTANT NOTICE: You must localize at least one file in your target in order to have this information about languages shown on the AppStore (as you are doing this on per target basis, you can have different languages shown for each target, good for white label apps). I usually localize LaunchScreen.storyboard as you usually don't have any text here.
In order to localize storyboard, you need to tap on it and in the right menu tap "Localize..." button:
Localize button
After that, just check the languages you want it to be localized to (in my example German.
Localized storyboard languages list