What I see is that your "banner" redirects to an ez-page. The default ezpage option of a ZC site (at least pre 1.5.3) doesn't exactly offer multi-language ez-pages. Further, when the ez-page was shown and it was attempted to change to spanish at that time an alternate language ez-page didn't show... I haven't pulled up the ez-pages admin area to try to help resolve this, but at least to prevent the white screen if you have the banner forward to the page/link you currently have and add &language=en to the end then at least there wouldn't be a white page, but there would be on english in the text. Otherwise, need to get the banner to be in spanish with it's own redirect to a spanish ez-page...
But it would be that much more helpful to answer torvista's questions. Oh there is a multilanguage ezpage add-on, but again, not sure if it is needed under 1.5.3 or not, I haven't looked to see if multilanguage issues with ezpages have been resolved without the add-in.