[ufmace]

Just go ahead and start building. You didn't mention a mobile OS, but if you know Java well, it should be straightforward to pick up Android development, which is already in Java. IMO, don't bother with any other languages, unless you really want to build the iOS version first. Just throw something together for the UI to get started and get the core functionality working. Then, if it feels too ugly, try working on that some.

You could maybe hire a contractor or something to help make the UI better if needed. Personally, I wouldn't bother unless your app is published and bringing in money, and you can make a business case that spending $X on making it better will bring $Y more revenue. That would give you an idea of what's a good amount to spend. I wouldn't worry about any of that yet though.

[coin]

Java to Swift is also very straightforward. Most of the learning is with OS’ API. It’s nothing a competent software person can’t handle.

[cliqueiq]

And also nothing a competent software person should waste their time on.