As best I can tell, as far as root causes go:
(please forgive me if I’m off base as I started using Brave yesterday)
This issue appears for users who are using the linux zip distribution, the source for arch/slackware/manjaro etc all appear to be built by third parties but based on the official zip distribution.
This issue does not appear in the official packages buildt for distros such as Ubuntu
Inspecting the zip distribution for dev version 0.60.16 and the corresponding Ubuntu package there are a number of differences. Several directories and files are omitted in the zip version. I don’t know enough about Brave to understand if they are all significant yet.
resources directory in the Ubuntu package is missing from the zip file.
resources directory is extracted to the same location as the official Brave zip has been extracted to and Brave is run this issue is resolved. (Shields has text and Rewards has the localised strings)
brave_extension resources and
brave_rewards locales seem to be missing or packaged in a way they’re not being sourced correctly and loaded in the zip distribution.
I am unsure how to fix this in the build.
There are a number of conditionals and nested scripts in the build (source: https://github.com/brave/brave-core/blob/master/BUILD.gn etc) and I can’t work out which line or file is wrong/missing to correctly include this with the zip distribution.
(edit: sorry my screenshots are jank, some hidpi issue, but you should get the idea - please let me know if you want further information)