According to Localization | Bitwarden Help Center
This appears only available currently on the web-vault, desktop, and mobile apps, with the extension language going off the browser’s settings.
Bitwarden browser extensions will dynamically change to use the language set by your web browser:
Hello there, are there any plans to allow users to manually change it? E.g. I need my browser in language 1 but the Bitwarden extension I want in language 2. At the moment I’d have to install a second browser just to use Bitwarden in language 2, not really a solution.