htaccess rewrite after/with redirect - .htaccess

Question

When there's 404 missing page error this URL is called:
https://domain.com/404/
Inside the 404 folder there is only a htaccess file that will redirect to language specific error pages (subfolders only with one index.html in them).
/
404
.htaccess
pt
index.html
en
index.html
htaccess file code:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
# If Accept-Language starts with 'pt',
# then redirect (only) subdirectory 'pt'
RewriteCond %{HTTP:Accept-Language} ^pt [NC]
RewriteRule ^/?$ %{REQUEST_URI}pt [L,NC]
# Otherwise, redirect to en
RewriteRule ^/?$ %{REQUEST_URI}en [L,NC]
</IfModule>
The final URL looks like this for Portuguese for example: domain.com/404/pt/
What do I need to change so that the URL will still redirect to the index.html in the language specific subfolder but will look like domain.com/404(with or without trailing slash)?
Thank you.

Answer 1

For silent rewrites to language specific 404 use this .htaccess in /404/ sub-directory:
ErrorDocument 404 default
DirectoryIndex index.html
RewriteEngine On
RewriteBase /404/
# If Accept-Language starts with 'pt',
# then redirect (only) subdirectory 'pt'
RewriteCond %{HTTP:Accept-Language} ^pt [NC]
RewriteRule ^/?$ pt/index.html [L]
# Otherwise, redirect to en
RewriteRule ^/?$ en/index.html [L]