I am a beginner in Xcode. I don’t have xcode 11 on my mac because it cannot run Mojave (required osx for Xcode 11).
Can I create the app on my computer in Xcode 10, then send the project file to a friend with Xcode 11, and then publish the app to App Store from my friend’s computer? What problems can I expect? Swift 4/5 incompatability? Is there any other solution?
Yes, you can. If your swift version is 4 it's okay. not a problem. I think the only thing will happen in your friend's Xcode is a warning that says to you 'Conversion to swift 5 is available'. But it is okay to ignore it and to publish your application with swift 4