I would recommend to do it yourself at this stage. I've found Amir Khella's UX course on Udemy to be very helpful: https://www.udemy.com/ux-for-founders/ It's a structured, methodical approach that most people are capable of applying. You won't beat the pro's of course, but it will be good enough to start with. In terms of UI, try to keep it simple and minimal for now. Consider starting from a good template. Less is more. Your main focus at this stage should be on validating the core idea of your app and iterating that to achieve product-market fit. That will be way more important that professional grade UI/UX right now.

I agree with Grant, don't outsource at this stage, at least not the core idea of your business. (e.g. you could outsource development of the marketing website) The single biggest advantage you have as a startup is speed. Large companies have long decision making processes, need time to align everybody on a new direction, often have heavier planning processes that require some kind of formal sign-off etc. As a startup, you can make quick decisions and change course in a matter of days. Don't throw that advantage out the window by outsourcing your core development.

It's never too late Jerome, though it will depend on how much time & energy you're willing to invest. Which skills are you lacking to build a successful app business and are you ready to invest the weeks, months, possibly years of diligent work required to build them? The mobile app space is extremely competitive these days, and frankly, ideas by themselves aren't worth much. As Derek Sivers correctly puts it, ideas are merely multipliers. Execution is worth millions. (Read http://sivers.org/multiply for more on that) To succeed, you (or rather, your team) will need the design skills to turn your ideas into a delightful user experience and the technical skills to turn that into stable, fast, scalable apps. Those things are merely table stakes though: the toughest nut to crack is sales & marketing. Who is your audience and how will you (profitably) reach them? Best of luck if you decide to push through with it, it's a fun ride!

This is a difficult question to answer without knowing more about the app you're building. There are tons of different platforms out there and the landscape is evolving rapidly. As a result, I don't believe anyone can claim to know all of them in detail, so a lot of it will come down to having a skilled development team. I have personally built full stack solutions in a wide variety of domains, languages and platforms for the last 15 years or so. In my experience, there is no single "best programming language": many of the popular languages will be up to the task. Similarly, there is no single "best app framework", though one will lend itself better to a specific problem than the other. The key to success, IMHO, is having experienced developers that you can trust and rely upon to make an informed decision and then see it through. Generally speaking, I tend to steer clear of cross-platform, "develop once, run anywhere" solutions, as that often solves for the greatest common denominator that doesn't feel quite right on any platform - exactly the opposite of what it takes to win in the extremely competitive mobile market. I'd be happy to jump on a call and discuss your concrete situation. The more details you can share about the type of app you're building, the easier it will be for me to provide specific recommendations.