Hi Carlos,
Thanks for reaching out.
This seems to be related to pre-installed language and not with WPML. If that’s the case, please deactivate WPML first, then please follow this https://www.hostpapa.ca/knowledgebase/change-wordpress-language/#:~:text=To%20change%20the%20site%20language,use%2C%20then%20click%20Save%20Changes… I have tried that and it works, please note that it’s for pre-installed language, an example is when you installed this https://es.wordpress.org/download/ and already contains Spanish language.
Unfortunately, we don’t provide site’s maintenance and this process is similar to it, it could destroy your existing translations. What I provided is just a guide, pre-installed languages or even newly added languages aren’t part of the theme, the theme is just translatable but it’s user responsibility to maintain site’s translations.
Thanks!