You can change the language of your Nextcloud web interface from the settings. To do this, start by clicking on the circular user menu in the top right corner and then select "Settings".
This will open the settings view, where you can customize your Nextcloud profile. From the highlighted section in the image, you can change the language of the user interface.
From the same view, you can also configure various aspects of your profile, such as your name, email address, and contact information. You can also see the current status of your storage space and the groups you belong to. On the left side of the screen, you will find additional settings menus.
For more instructions, you can refer to the official Nextcloud documentation available at: https://docs.nextcloud.com/server/stable/user_manual/en/index.html