Ask HN: Making a mobile app as a back-end developer?

My main area of expertise has been backend java development for years. I have a few mobile app ideas I have been designing the backend for a while. But since I have no frontend experience, I don't know how to proceed. Should I start learning, which will take large amount of time since there are plenty of platforms you need to publish, or should I outsource it somehow ? If outsourcing is the right answer, where to find people I can work with ? Please forgive my English.