Default language reverts to null

I installed Grav today ( Grav v1.6.23 - Admin v1.9.13). I'm trying to change the default language to Portuguese (code pt). In the admin plugin I added pt to the languages supported with no problem. However, when I tried to set the default language in the admin plugin, after saving the page, the pt code still appeared in the input field, so I thought it was fine. But the site didn't change to Portuguese, so I looked at the user/config/system.yaml file. The default_lang was null. I changed this and saved it and went back to the site. It was still not in Portuguese so I checked the system.yaml file and it had reverted to null. I tried several times and the default_lang always reverted to null. What should I do?