[Risolto] I can't add Spanish language

Background of the issue:
I am trying to add Spanish to my languages in WPML.

Symptoms:
I expected to see Spanish added, but instead, I got nothing.

Questions:
Why can't I add Spanish to my languages?
What steps should I follow to successfully add a new language in WPML?

i tried to add it manually also with "edit languages" but it says that it already exist