If you don't select a language, then it's Undetermined. It'll be visible to those who allow content in the Undetermined language, and most instances' default settings include Undetermined because if you unselect it, you won't see many posts whose users didn't pick a language.

I have both Undetermined and English in my account setting

This determines what content you can see. So it means you will see content tagged with English as well as content that doesn't have a language tag.

each post and comment have those two options but i leave those it on “Select language”

This means that your posts and comments are not setting a language tag when you make them, so people who are able to see content without language tags will be able to see them.