App Store or wherever you deploy your App.
Do you need to support multiple platforms? If you need your app to have both a mobile and web component, selecting a cross-platform development tool and language, like Xojo, for example, is ideal so you can share code and save time. Also, if later down the line you need to support additional platforms, cross-platform tools make it easy to do so.
That a language is new or modern might sound great but if it is not well-suited for you or your intended purpose, the fact that it is new or modern won’t matter. What’s important in a programming language is that you are productive with it. The best programming language for you is one that lets you quickly and easily:
Whatever programming language you decide on, if you find that it isn’t right, move on. As we mentioned, a lot of the basics are the same so you should be able to apply the skills you have developed to learn a new programming language.