I’m going to put the answer here that I put in the Nemeth forum as this question was asked in both places.

As the literary text of a math book is done in UEB, you would follow UEB rules for the Foreign Language. If you happen to have the Spanish word “y” within the Nemeth, put a single-word switch indicator on it and treat it as you would if it were within the literary text (just as you would with the English word “and”).

Cindi Laurent