Hello,
I am interested in purchasing the domain piensaenlinux.com
that seems to have been purchased (after expiration) by a domain
catcher that has put japanese porno content :)

Anyway I have been trying to contact unsuccessfully to the
email listed in Whois: admin-email: webmaster@mail.ne19.net

So I was wondering if someone here could help me.

Well it might be the way you are asking? It might be the owner doesn't speak english. Try translating you request with Google translate into Japanese. Include both the English & Japanese in your email. I think you should mention the price you are prepared to pay for the domain, as well.