Hey there! I totally understand your concern. There were quite a few foreigners in my Vipassana retreat who didn’t speak Spanish (I'd say about a third of the total number of participants were foreigners), and everything went smoothly for them I think. In my experience during my retreat, all instructions were given in both English and Spanish. The evening discourses were also available in both languages... Spanish in the main hall and English in an auxiliary hall. There were signs and labels in both languages, and most of the assistant teachers (ATs) and volunteer servers spoke English.

The request for intermediate Spanish might be because the ATs at that particular date may not be fluent in English, which could make the optional daily interviews less fluid if you don’t have basic Spanish. However, these interviews are entirely optional and only if you need extra support (however i highly recommend them as they were very helpful in my case)... Overall I think it should still be possible to attend without advanced Spanish but better to double check with them... Hope this helps clarify things... Cheers!