Como se dice "Welcome Back" en español?
Some people will disagree with me, but this is what I think: there is no proper translation to that in Spanish. We welcome people whenever they arrive, but we do not have a simple way of express "You left and now that you are back, we welcome you again". If you think it is weird that we do not have that, I can give you hundreds of examples of things in English that are extremely weird to all non-English speakers of the world, and you take for granted. "Bienvenido de nuevo" can be said, but it feels unnecessary and even silly in our language. We say things like "¡Oh, ya has vuelto!" (Oh, you're back!), but we do not have a phrasal verb like you do to say this. English is not the language of the gods; we do not have to speak like you do; your language is not the canon of how people should speak. In Spanish we do not say "welcome back", but "Welcome again", and it sounds funny, if not ironical.
Again, I can think of hundreds (if not thousands) of things that we say in Spanish that are untranslatable into English, so just accept it: languages are different.
Now, go and translate the Mexican "Ándale" properly into English, if you can.
Maybe I'm completely off base, but I think all that is necessary is bienvenido. For example, if your friend went on vacation and returned, if you told your friend bienvenido he would understand you were welcoming him back from his vacation. I've never heard anything different as it regards welcoming someone 'back'.
La forma más correcta de traducir "Welcome back" es "Bienvenido a casa" cuando a alguien se le dice "Welcome back" significa que esa era su casa, ya se refiera a su lugar de trabajo, escuela, o equipo deportivo, etc. Ya lo saben "Bienvenido a casa".