That’ a great question! This would largely be dependant on the exact apps you are looking to build, as different apps can do different things and fill different parts of your workflow. Mainly:
Board Views/Widgets - those would require a way for the programming language to visualize your data. Usually, this is done through using React, or other JS frameworks.
Integrations apps - those can be built using a variety of programming languages, like Python, PHP, JavaScript (Node) or others that can handle running a server, as well as receiving data and sending data.
I hope this helps clarify a bit further, although if you could provide more specific details, that could also help us point you in the right direction