Based on the your input, I would suggest using URL rewriting configuration as follows:
RewriteEngine on
RewriteRule ^([a-zA-Z-_]+)$ $1.php?lang=en [R]
RewriteRule ^([a-z]+)/([a-zA-Z-_]+)$ $2.php?lang=$1 [R]
The first rule rewrites URLs like /abc (with no specified language) to /abc.php?lang=en and sets English (en) as a default language
The second rule rewrites URLs like /xy/abc (with specified language) to /abc.php?lang=xy (language is taken from the first part of the request URL)
Examples:
http://mydomain.com/en/references => http://mydomain.com/references.php?lang=en (RewriteRule #2)
http://mydomain.com/ro/referinte => http://mydomain.com/referinte.php?lang=ro (RewriteRule #2)
http://mydomain.com/references => http://mydomain.com/references.php?lang=en (RewriteRule #1)
http://mydomain.com/referinte => http://mydomain.com/referinte.php?lang=en (RewriteRule #1)
Please note that the rules will ignore requests for specific PHP files i.e. requests as follows: http://mydomain.com/ro/referinte.php or http://mydomain.com/referinte.php will not be rewritten. Also, server response should be the same for non-existing pages as currently i.e. assuming /referinte.php does not exist, the requests as follows: http://mydomain.com/en/referinte (new rewriting) and http://mydomain.com/referinte?lang=en (current rewriting) I presume would return 404.
I hope that will help.