RewriteRule ^en/ - [L]
RewriteRule ^de/ - [L]

RewriteCond %{HTTP:Accept-Language} ^de [NC]
RewriteRule ^ /de%{REQUEST_URI} [L]

RewriteCond %{HTTP:Accept-Language} ^en [NC]
RewriteRule ^ /en%{REQUEST_URI} [L]

Just curious, how does it work, have you tested these by any chance?
Also – These different multilingual websites will their own .htaccess files, in which of the .htaccess files is the above code to be written?

Thanks!

The first two rules prevent the other rules from being used when a specific language has already been requested. When browsers send requests to your server, they send a header that tells servers their preferred language. The %{HTTP:Accept-Language} variable is assigned whatever value the browser sent. The second pair of rules rewrites the request to the /de/ folder when the preferred language code begins with “de”.

Same goes for the last pair when the preferred language code begins with “en”. “en-US”, “en-GB”, “en”, etc. all will match the ^en parameter.