Is it necessary for a developer to upload an app written in vala only? Can't it be something c++ or python to be uploaded on Elementary's app store?
I really want to contribute for them but don't want to make a language shift.
This is answered in the Developer Documentation:
Your app may be written in any language, but the front-end must be a native Gtk3 app. Web, Electron, Qt, Java, and other non-native app front-ends will be rejected during the review process. A game may be excepted from this requirement so long as it uses native window decorations and is generally usable on both loDPI and HiDPI displays.