Why though? Why would you want your UI code in Python, as an example? Python has nothing in its standard library to help make websites. Tools like PyScript might give you the DOM, but suddenly you'll be using shims for all the other stuff you need, like resize handlers or fetch() or Intl.NumberFormat or anything else you care about.

Web apps are JavaScript. Android apps run on a JVM. iOS apps are whatever can compile to native code and use Apple's APIs. When you avoid the runtime of the language that you're expected to build for on a platform, you'll inevitably spent most of your time working just to get a handful of features that you need from the runtime into the language you're choosing to use.