Something

RewriteEngine on

RewriteRule ^([a-z-]+)/?$ index.php?lang=$1
RewriteRule ^([a-z-]+)/([a-z\_0-9-]+)/?$ index.php?lang=$1&cat=$2

RewriteRule ^ajax/([a-z-]+)/?$ module.php?lang=$1
RewriteRule ^ajax/([a-z\_0-9-]+)/([a-z-]+)/?$ module.php?lang=$1&cat=$2

RewriteRule ^admin/([a-z-]+)/?$ admin.php?lang=$1 [L,QSA]
RewriteRule ^admin/([a-z-]+)/([a-z\_0-9-]+)/?$ admin.php?lang=$1&s=$2
RewriteRule ^admin/([a-z-]+)/([a-z\_0-9-]+)/([a-z\_0-9-]+)/?$ admin.php?lang=$1&s=$2&m=$3
RewriteRule ^admin/([a-z-]+)/([a-z\_0-9-]+)/([a-z\_0-9-]+)/([a-z\_0-9-]+)/?$ admin.php?lang=$1&s=$2&m=$3&c=$4

<Files ~ "\.txt$">
    Order allow,deny
    Allow from localhost
    Deny from all
</Files>