[IceSentry]

If you already know java for the backend then just use that. Or if you want to learn something new pick one language. You don't need to learn all of them.

For the client side I would recommend vue, but react is also very popular.

Once you have chosen the backend/frontend technologies then you can start building. If you don't know anything about rest learn how your apecific framework does it. You don't need to learn rhe whole spec. At least not in the beginning.