Is it worth attending a zarzuela performance if one doesn't speak Spanish? We will be in Madrid in late March 2020 and the performance we would see is "Policias Y Ladrones" (Cops and Robbers), which seems to be a slapstick/farce thing--possibly funny and entertaining independent of language.
I would say go see it, seems like a unique experience :)
Think of it as an Opera, I don't understand much of German myself, but I still enjoyed Mozart's Die Zauberflöte aka The Magic Flute. I'm sure the physical comedy aspect will make it more interesting too.
Besides, from the articles online about this new Zarzuela, it seems you need to be "in the know" about the current corruption and political situation of Spain too, so one may not understand a thing, even if one fully speaks Spanish!
It could be fun trying to guess the story or even making up your own story, to go with the action lol :)