If you have the file /layouts/404.html, Hugo will create the /404.html page for you. However, in a multilingual setup this page will go into each language subdiirectory, so you get /en/404.html, /ru/404.html etc.
My problem is that render.com only recognises a 404.html as the default error page when it’s at the root of your site.
How can I have my 404 page created at the root, instead of under each language subdirectory?
It’s a hack, but does what you need. I don’t believe that you will be able to tell Hugo to create only one single 404 page for multiple languages. (public/en/404.html depends on your structure of course)