How can I change and, with that, the language of my page either with a language selector or automatically all the text, to the selected language.
I have seen that .js or Locale language files are downloaded with angle, but only changes some parameters such as currencies, dates, etc.
Good, I do not know if I understood correctly. Do you want the user to be able to choose between several languages on your page? I honestly would do it with "hard coding", that is to say, it would offer a home page in which, first of all, the user has to choose in which language he wants to see your page. For example:
index.html:
"Select the language" [Spanish] "Sprache auswählen". [German]
And depending on whether the user has selected to redirect to an index_es.html or an index_de.html.
Another alternative would be to use as you mention angular translate. Here is an example of how it would work: Angular Traslate