There are few ways of editing or translating a 404 page:
1. Changing a 404.php file in your theme folder:
NOTE that all modifications should be done within Child Theme.
2. Changing default English strings. Follow this tutorial in order to get more information:
https://support.crumina.net/hc/articles/13/25/228/change-default-english-strings
NOTE: Save your changes in the child theme!
If you want to set a custom design for 404 page there are a lot of plugins can help you with changing page, for example, 404 Page plugin, Redirection, 404 page by SeedProd etc.