Cannot switch to another language

I tried yarn start, and tried to switch to another language, but as the picture below, page cannot found.
<img width="1285" height="359" alt="Image" src="https://github.com/user-attachments/assets/8a38d78e-bd85-42c5-9fc3-3068dcc52e98" />
Also, I noticed that switching to English is just simply append /en to the url.