If you set your calibre language to pt_BR and have a pt_BR.po file in the current directory, calibre will try to use it as the language file -- this is to help translators who are translating calibre itself to debug their trnaslations files easily. See
https://calibre-ebook.com/get-involved for details.
You can use the following to prevent that error message (which is harmless)
set CALIBRE_OVERRIDE_LANG=en