The first is written in a language that you know, and after that if the app "took off" see what the rest(resources/capabilities) and either improve their code or rewrite in a language more suitable for the tasks, or move to more powerful hardware.
In 99% of cases the interaction between the client and server implement using standard protocols(http(s) WebSocket protobuf, etc.), libraries for working with them are in all the popular languages.
Well, since you wrote the sites you should understand that the frontend of the website is essentially a client to the backend and it's easy to replace, even a mobile, although desktop client without any changes of server code. Of course the server code itself should provide the API, not to be a shitload of PHP+HTML.
As for the hosting to start, you can use any, even shared. But it is better to take vpsку, for example in Aruba for a total consideration of € 1, for 3евро OVH or DigitalOcean for$ 5. But at design time you can use the free plans from Amazon/Google/Microsoft.