I would likely list all the casinos I like, emphazise the Spanish ones, and seperate them out making sure everyone knows whether they have Spanish support or not.
But that’s just me, you may be better off strictly exploiting the Spanish niche, no telling until you get it going for a few months.
The thing with my site is that it already had a lot of international traffic before I added the specific language sections, and it has a lot of repeat visitors. So the Spanish players I have may well be ones that have played through me all along and don’t need the Spanish language.