Force language segment in URL on multi-lingual site
Make sure to remove Craft’s default rewrite rule (“Send would-be 404 requests to Craft”) from the .htaccess in your document root, so it’s only in the .htaccess files of your locales’ subfolders. You can now remove the index.php as well and requests to your homepage domain.com/ should now respond with a 404 HTTP error.

You have now removed all (duplicate) routes starting at root level.

Last step is to redirect visits to the homepage to en/ via the .htaccess in your document root.

RewriteEngine On
RewriteRule ^$ /en [R=301,L]