“How art thou” is slightly older English for “how are you”. In Spanish you have the distinction between the intimate “tu” and the formal “usted”. The same exists in English as “thou” is the equivalent of the “tú” form. So when you say “how art thou (art = are) you are saying ¿cómo estás?.
The majority of English people no longer use “thou”, although you will see it in writing and hear it in classical plays and films set in the past.
Use the phrase on your English friends, and if they look at you in a puzzled (perplejo) way, then you can have the pleasure of explaining the history and the difference!.
