Unless the application you're planning to distribute is very huge or is using a cross platform framework that makes it a nightmare to support multiple localization in an app, the best solution would be to just include all the localizations in your app and support files.
There is not really a way to support what you are tying to achieve with the localized text files. At best, you could create a custom installer plugin to display the ReadMe file in multiple languages but this would still not allow you to do the same with the Welcome (and Conclusion) steps. And synchronizing the language selected in the fake ReadMe step would not be easy or possible.
Allowing to install a different version of the app and some support files depending on a selected language could be possible using choices and using the custom installation mode by default. But again, there's not really a way to synchronize the selected language in the License pane (or your custom fake ReadMe pane).
Honestly, your 2 best options are:
when in macOS do as the macOSians do: let the system language settings decide which localization to use for the app and the Welcome, ReadMe (and Conclusion). And have an application bundle/components that include all the localizations
distribute 1 installation package per language.
I don't believe that merging the Welcome, ReadMe and EULA in a single file and use that file for the License step is a user friendly solution.