Please help us help you and fill where relevant:
Your LimeSurvey version: Version 5.3.32+220817
Own server or LimeSurvey hosting: own server
Survey theme/template: Vanilla (default) Bay of Many (Admin) 
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Hi, I have two themes, Vanilla and Bay of Many. 
Survey is on Spanish, we have a problem with the registration template, that we see english information (like First Name, Last Name, etc.) and we need to change to spanish. But we didn't saw where?
Administration is in english and we have the same problem, where we change the language? 
We need to install themes in spanish?


Thank you.

Most probably these strings have not been translated by anyone yet.

As there is a number of "Spanish" versions, which Spanish are we talking about?

The backend / administration can be switched to Spanish during login, there is also a general setting for the main language and then each user can set the language in their user profile.

See here:  translate.limesurvey.org/projects/
For Spanish (Spain) for example the strings are translated:

 

For Spanish (Chile) and Spanish (Mexico) it also seems to be translated, but Spanish (Argentina) (informa) looks like this:

 
So I guess you are using Spanish (Argentina). The problem seems to be that no one in the Argentinian community has yet made the effort to translate those strings.
 

Thank you, if I use Spanish (Spain) works fine

Ok if I use Spanish Spanish works fine, in the meantime I can translate Spanish (Argentina).
I saw another translations (like Spanish Mexico) that I could use like a pattern because the are very similar. Have I some way to import this translations to us?

Read the manual - manual.limesurvey.org/Translating_LimeSurvey