If you want to contribute to this App through writing Code, you can do so. Just contact the Administrator or create a new Issue stating that you want to contribute. The Administrator will answer you with the spefific details.
You should have a bit of knowledge in Swift, SwiftUI, Apple's design language, the Human Interface Guidlines (HIG) and App Development in general. Also it's advantageous to have knowledge in using Git and Github.
You should at least speak english, because the default language in this App is English.
If you cannot or don't want to write Code, but still want to contribute to this App, you can do this by reporting Bugs and Security Risks. Another way to contribute is to suggest new Features for this App.
Just create a new Issue and the developer will take care of it.