Tip on how to create a website in 2 languagues

Hi, I wonder how to create a website that is for example in ENGLISH and then, a mirror website in Spanish, without creating 2 websites with 2 domains, iinked? Have you seen those websites where you just click a button, which can even be one of the menu bars, and then you see exactly the same website but with the info in english?? How they work that out? using a subdomain or how?

Thanks for your help. I am new new on this.