This should work
RewriteEngine On
RewriteRule ^([^/]+)$ http://www.domain.com/lang=$1 [R=302,nc]
Change R=302
to R-301
when you are sure the redirect works.
EDIT:
RedirectMatch 302 /jp http://www.domain.com/lang=jp
RedirectMatch 302 /cz http://www.domain.com/lang=cz
RedirectMatch 302 /eng http://www.domain.com/lang=eng
RedirectMatch 302 /en http://wwwdomain.com/lang=en