We downloaded Loco Translate but thats as far as we got, we searched for the 404 page and got the follow results,
404 page
404 page content
Use it to change the content of the 404 page. Use HTML. Do not include JS.
Of course we never used this plug in nor edit are translation files before so we do not know how to do this. http://prntscr.com/oqqr5m