Custom 404

I followed Bob’s instructions for finding the 404.html page found in this thread but could not find ‘404.html’ – only ‘404.hbs’. Can I simply replace ‘translate error.message’ and ‘translate error.404’ with custom language?

I’m using the Mercury theme.

Thanks in advance.

{{> head}}
    {{> top}}
    <main>
        <article class="post u-wrapper">
            <header class="hero">
                <h1>{{ translate 'error.404' }}</h1>
            </header>
            <p>{{ translate 'error.message' }}</p>
        </article>
    </main>
    {{> footer}}