You will need at least 2 stacks. Python and Javascript. (Could be other combinations as well like Python and C#)
Python programming would be ideal - but this is in as far as having a web based, desktop application that can run on windows and linux. When it comes to mobile - you will require another stack as well - a cross platform mobile development environment comes in handy then - e.g. react native or Flutter.
The way I look at it - at least 2 stacks - One that gives you the api, web platform, desktop apps - Python can do this - then the other is mobile interfaces - where react native or flutter can give one codebase that can be compiled to Android and iOS apps