Please help us help you and fill where relevant:
Your LimeSurvey version: 5.6.23
Own server or LimeSurvey hosting: limesurvey
Survey theme/template: fruity 
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Hi there!
I opened my survey for public registration, so now people can register using the registration form. I would like to edit the text shown (see attachment), as I don't like the Spanish translation. Can I actually do that by editing CSS in my theme?

Thank you in advance!
Lourdes

I think, the easiest is to edit the "register_message.twig" of your extended theme.
 

as I don't like the Spanish translation

If your translation is an improvement, you can suggest the change at translation.limesurvey.org (same credentials as in the forum) and can help to improve Limesurvey.

Of course, it needs to be close to the English base and can't be something very specific that would just work for your case. It needs to be generally usable to make it into the translation files.

Great, thank you!