Hello,
Welcome to Microsoft Q&A!
To add multiple language support to your app, it's better to do that when you are developing the app instead of after packaged the app.
If you are developing a UWP app, you could follow this document Globalization and localization to add more language resources to your app.
If you are not developing a UWP app but other kinds of apps like WFP apps or Winform apps, then you might need to change tag and look for help for other teams.
Thank you.