About Custom Keyboard in Swift

Unfortunately, there is no API for Japanese conversion, so you have to implement it yourself.

An easy way to do this is to use the Japanese conversion web API, such as Google Japanese input API.In this case, network access is required for Japanese translation.

https://www.google.co.jp/ime/cgiapi.html

Another not-so-easy way is to build an open source version of Google Japanese input for iOS.

(Example)
https://github.com/kishikawakatsumi/Mozc-for-iOS