Welsh for Spanish?
It would be interesting to find out if there was significant demand for such a course, given the significant Welsh-speaking community in Patagonia, and perhaps the US Hispanic members of Duolingo might also attempt such a course. Certainly, the reverse course would be viable in Wales!
Such a course would be great for both the Welsh speaking community of Patagonia and also for the many Welsh medium schools teaching Spanish.
I would like to learn spanish from Welsh and as rmcode says it would be a useful resource for schools in Wales - Welsh or English medium. Even if english is your first language, I have heard that daisychaining languages is recommended by a lot of people who seem to have the knack of learning lots of languages.
I think if a Welsh person wanted to learn Spanish they would do the Spanish for English course. I'm pretty sure most Welsh people know English.
They could do the Spanish for English to learn Spanish but the OP mentioned a Welsh for Spanish course to learn Welsh.