RewriteCond %{REQUEST_URI} !(/(en|gr|ru)) [L]
RewriteRule ^(.*) /en/$1 [R=301,L]
RewriteCond %{REQUEST_URI} ^/$
RewriteRule ^(.*)$ /en/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -d [OR]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]
RewriteCond %{REQUEST_URI} !^/(?:ru|en|gr)/
RewriteRule ^([^/]+(/.*)?)$ /en/$1 [R=301,L]