This bugged me as well. I've found a workaround.
Go to Library/Application Support/Orion/Defaults/Extensions/.
There you have all the extensions (both FF and Chrome) in folders with encrypted names.
For each extension go into the folder and open the manifest.json file.
Search for the line "default_locale": "en", and change it to the locale of your choice. For me de for German works fine for AdGuard and Bitwarden (tried FF and Chrome extension).
If an extension gets an update it will probably set back to 'en'.