That is a good find, I have just done a test on my local instance and I noticed an error in the browser console. I have let the team know internally and I will let you know what the outcome is.
As far as I know is that expected behavior as we do not have any translation files for any language except English. So no matter what you set it to will it not find any files and then fall back to English.
thank you,but I am not familiar with the frontend, so I am based on the docker container. I found three places inside the container. Is it possible to modify the translation inside?