Language of Sign up links

[Inka]

Hello,

How can I make the sign up screen of a specific plan to show in a specific language?

For example, this is a sign up link:

http://cp.mydom.biz/psoft/servlet/p...11&signup=yes&template_name=signup/begin.html

What should I add to the query string so that the sign up screen is shown in (say) spanish?

Thanks in advance