When you develop a new website using Umbraco, you may notice that the 404 page is intentionally left ugly and you need to make sure it does have some lovely template.
It is possible and straightforward to create a great 404 page in Umbraco using just Umbraco CMS. It means that you do not need any web.config changes like you usually do in ASP.NET MVC application, Umbraco already has a build in support for 404 error page.
To create a 404 page for your Umbraco website you need to create a new document type (in fact you may use existing document type but point it to the different template).
Next, you need to update 404 template and add all required styles and content there.
After the template is ready, you need to Create a 404 content page and make sure it is not visible in the navigation.
After you saved the page, you are going to get a page ID, which you can find either in URL or on Info tab like on the screenshot below.
With that ID, you need to open UmbracoSettings.Config file and update <error404> tag with the ID of your page.
As you see from the documentation, you can assign different 404 pages for different languages
Now if you try to access any page which does not exist you are going to get a friendly 404 page which using your template
Also, Umbraco also taking care of response code which browser get back, it is a correct 404 Page not found code, so you should not have issues with search engines.