Thank you for your answer (Google rocks ;) ).

I want wiki users see English texts without be logged.

I tried with several browsers on two different computers, and I see always Spanish texts. If I tell my users to log in the wiki, they are to access a form where they see "Tu nombre de usuario", "Tu contraseña", ... very confusing.

It says (gtrans) you have to be logged-in in order to change your preferences..