Thanks!!!
Finally I could do it by creating a “en_GB.po” file and locating into wp-content/languages/

Thanks!
But, instead I hace new problem related with this one:
I will try to explain my best.
Before I did this changes, I wrote in english the text in “Popup text
Place here some information about cookies usage that will be shown in the popup.”
By WPML I could translate it to spanish, because the string translation appeared.

But after changing the .po file I am trying to translate that text trough WPML plugin but the string translation has gone.

And, another bad thing is:
When I go to Theme Settings, and select an option for english (for example: “Popup text
Place here some information about cookies usage that will be shown in the popup.”), writing “Cookie Policy Information , and after that I click spanish in order to translate it (into “Información de Política de Cookies”, the thing is that the text appears in english and if I translate it into spanish, this translation is automatically set in the english theme settings….

If you need, I can give you admin credentials.

Please, could you help me?

Thanks a lot!
Estefanía.