The website should be shown in a system language by default

[ryonakano]

(Sorry if you project members wanted me to post issues on https://git.joinplu.me/plume/joinplu.me. If it's more suitable place feel free to close this issue and open the same issue with it there 🙇‍♀️)

The joinplu.me is also translated in many languages but the website does not seem to be opened in a system language unlike Plume itself. Users need to find and open the top-right menu to show the website in their language. It would be nice if the website is shown in a system language by default.