Re: how to pass a paramter to xsl in a simple way?

I have xsp documents with a xml structure for two languages,

This seems to be a Cocoon question.
You should take a look at i18n transformer, and a closer
look at the documentation of the XSLTransformer, IIRC
there is a use-request-parameter configuration setting.

For details, ask on the cocoon-user list, but be sure to
study the docs closely first.