Web developing/mobile app help

I was wondering what language would be good for coding an ecommerce app to go along with a clients website. Something like eBay or amazon
Also are there any special skills or techniques I need besides being proficient in the language. I’m more focused on android phones ATM. I heard java is the way to go…
I’m starting a web development agency. Focusing on websites for now. But I plan to incorporate app making where the client would basically tell me what they want and I would build it.
Any insight would be greatly appreciated
Also feel free to give any other advice regarding coding mobile apps(mainly android as I understand u need a Mac to use swift) and web development