Thank you, Chouby!! But I must say I already found the answer: when you edit the original language post (Spanish in my case) you find a box on the right upper corner for the English post ID. If you click the “Add new” option on the right of that, an ID is automatically assigned to the original post, which can be seen in the box.
Then you just need to copy that ID, edit the previously imported English post and fill the ID box with the number you just got, to tell WP this imported post is the translation of the original post. Et voilá!