As far as load on your server this will really depend since how many users/how much traffic a server can handle is very dependent on the software you are running and the activity the users are generating.

Building an app to work for multiple countries is a challenge and there are several ways to meet it.

Supporting multiple languages is usually used by creating a language file (or db table) where resources are mapped to an ID and language + ID can be used to return the appropriate string.

Ensuring low latency for users around the world can be accomplished either by hosting services around the world or by using a CDN or service like CloudFlare which proxies your app throughout the world.

Can you run an app for multiple countries from a single database, absolutely. As you grow, most db servers have methods for scaling across multiple servers allowing you to serve more users from a single database.