Spanish site, some words translation still in English

Basically, what’s happening is that whatever plugin/theme you are using “forgot” to translate those strings.

Translations in WordPress are handled by Gettext, you can learn more about it here and here.

If you really want to get your hands dirty and perform manual fixes to those untranslated strings you would need to find the string in the corresponding .po file and then recompile the .mo file, which you can do with an app like POEdit.

This also means that you would need to know how to find the corresponding .po file by understanding the WordPress file structure and doing a bit of research on how said plugin/theme implements its translations, which is out of the scope of this question. Of course there are other alternatives, like using a plugin like Loco Translate or similar, but plugins are also out of the scope of this question.