Translating Site to other languages

I have a client who is asking about making his site available in Arabic, Spanish and a European language. I can see that there are a ton of plug ins to translate the language text, but what is the best work flow.

Does a viewer chose a language when they land on the site?
Does the plug in sense the language the user is using?
Do I build three or four different language versions of the site?

Any advice or direction would be very helpful

Thanks!!!