RewriteEngine On

RewriteCond %{QUERY_STRING}  ^$
RewriteRule ^index_el.php$ /el/? [R=301,NE,NC,L]

RewriteCond %{QUERY_STRING}  ^$
RewriteRule ^index.php$ /? [R=301,NE,NC,L]

RewriteEngine On

RewriteCond %{THE_REQUEST} s/+(?:index_?)?(.*?).php[s?] [NC]
RewriteRule ^index /%1 [R=301,L,NE,NC]

RewriteRule ^el/? /index_el.php [L,NC]