Haven't had the problem but then again don't have a server that apparently uses pt_br_utf8. Thanks for error screen, but the tail end of the first line I think is key and might lead you to a fix ... expects lang type of what?
Since unoconv actually tries to talk to LibreOffice, the language issue might be with LibreOffice.
https://www.libreoffice.org/download/libreoffice-fresh/?lang=pick
If that's the case, then your best source for assistance might be with the aurthor of unoconv (really a python script).
https://github.com/dagwieers/unoconv
'spirit of sharing', Ken