@luison Thanks for reporting this issue. As per https://help.disqus.com/en/articles/1717203-multi-lingual-websites, Disqus only support languages which are mentioned on https://www.transifex.com/disqus/disqus/ and it doesn't have any support for 'es' language code. I don't know how we can suggest to Disqus to include support for more languages but we can modify the logic here in Drupal to always pass correct language code to the Disqus JS file.
Maybe we can create a configuration where user can provide the language code they want to set for all the enabled languages in the website. But that might be too much of work here. I am thinking of updating documentation or providing a way of altering the language code so that we don't get 404 errors.