Both the English and Portuguese versions of the generated page use the same RSS link from the host root instead of the language path. For example, on your demo site, when in English, the RSS link is "https://clente.github.io/hugo-bearcub/index.xml". It is the same when you are using the Portuguese site instead of "https://clente.github.io/hugo-bearcub/pt/index.xml".
Both the English and Portuguese versions of the generated page use the same RSS link from the host root instead of the language path. For example, on your demo site, when in English, the RSS link is "https://clente.github.io/hugo-bearcub/index.xml". It is the same when you are using the Portuguese site instead of "https://clente.github.io/hugo-bearcub/pt/index.xml".