Let's assume that the document root is /var/www/mysite and our website is https://www.mysite.org, and that the site has each translated sub-site under /var/www/mysite/en, /var/www/mysite/de, etc. (so https://www.mysite.org/en, https://www.mysite.org/de, etc.).
We write a rewrite rule to force a redirect from mysite.org/ to mysite.org/<language>/, where language is the 2-letter ISO code of the language, passed by the browser's "Accept-Language" header. We also write a rewrite rule to force a redirect from mysite.org/index.html to mysite.org/<language>/index.html.
<IfModule mod_ssl.c>
<VirtualHost *:443>
DocumentRoot /var/www/mysite
ServerName www.mysite.org
# choose site based on language
RewriteEngine on
RewriteCond %{HTTP_HOST} mysite\.org [NC]
RewriteCond %{REQUEST_URI} ^/$
RewriteCond %{HTTP:Accept-Language} ^.*(de|es|fr|it|ja|ru|en|zh|pt).*$ [NC]
Rewriterule ^(.*)$ https://www.mysite.org/%1/ [L,R=301]
RewriteCond %{HTTP_HOST} mysite\.org [NC]
RewriteCond %{REQUEST_URI} ^/index\.html$
RewriteCond %{HTTP:Accept-Language} ^.*(de|es|fr|it|ja|ru|en|zh|pt).*$ [NC]
Rewriterule ^(.*)$ https://www.mysite.org/%1/index.html [L,R=301]
# other directives here
</VirtualHost>
</IfModule>
Ideas taken from here.
