RewriteEngine On

RewriteCond %{HTTP:Accept-Language} ^ru.*$ [NC]
RewriteCond %{REQUEST_URI} ^/$ [NC]
RewriteCond %{QUERY_STRING} !(^q\=) [NC]
RewriteRule ^(.*)$ /ru/ [L,R=302]

RewriteCond %{REQUEST_URI} ^/$ [NC]
RewriteCond %{QUERY_STRING} !(^q\=) [NC]
RewriteRule ^(.*)$ /en/ [L,R=302]