Web site prefixes

first of all sorry for my ignorance on this topic but I'm getting started in the "web design world" and I pretty much need to know.

When you have your own site domain, say "www.example.com", and wanna add the chance to switch the language, "es.example.com" or de.example.com, or even add new features like a forum or a wiki, "wiki.example.com"... how would you do that? I mean, you can always add a subdomain, such as "example.com/es" but how would you do that with a prefix instead? Another example is when redirecting to mobile sites, the prefix "m.example.com" rather than "example.com/mobile".

I don't know if that counts as a new web domain but i guess it doesn't.