Plenty of docs explain how to change the languages preferences for browsing.
Some docs explain how to change the Brave user interface (UI) using the three dots option in languages dialog, but it’s not working on Linux.
Here is a simple way to start Brave user interface (UI) in many languages.
In a terminal type
( export LANGUAGE=my-language-for-brave-UI ; brave-browser & )
You may choose my-language-for-brave-UI among multiples languages, I tested fr,de,es,it
( export LANGUAGE=de ; brave-browser & )
( export LANGUAGE=it ; brave-browser & )
( export LANGUAGE=fr ; brave-browser & )
( export LANGUAGE=es ; brave-browser & )
NOTA 1, it’s not a way to have simultaneously Brave in multiples languages
NOTA 2, close all your Brave windows instances before to run the command