Hi,
Thank you for all the details, I think I understand what the issue is, I am sorry if I missed something.
If I get it correctly, you want to display only the items from the selected language, when one of the checkboxes (or both) are selected, but the problem is, that you see other posts as well, that does not belong to that filter. And this happens to any of the filters.
If so, then you are looking to disable this option right here: https://i.imgur.com/TfyDMn0.png
Basically what this does is, that when any of the filters is selected, then nothing else is displayed, only items that have any connection with that taxonomy. For example, if ‘English’ is selected, then anything that is tagged with ‘English’ will be returned, but nothing, that has no language assigned.
I hope this helps!