I tested this in Debian/Apache2 in vhosts file:
RewriteEngine On
RewriteCond %{DOCUMENT_ROOT}%{SCRIPT_FILENAME} !-d
RewriteCond %{DOCUMENT_ROOT}%{SCRIPT_FILENAME} !-f
RewriteCond %{REQUEST_URI} !^/en-US/hello\.htm
RewriteRule !^/[a-z]{2}-[A-Z]{2}/ /en-US/hello.htm [R=301,L]
that works.
/en-US/hello.htm must exist, otherwise, an infinite loop of redirection is possible.