I hadn’t realised that the library in Navarredonda was following a long and honourable Spanish tradition with their sign listing their rules and regulations for behaviour.

Presumably, though, the residents don’t want visitors to think that they are quite so stuck in the past as a ban on blasphemy and the image of a pony parked in the bull ring might lead you to believe. At least, I assume that’s why they felt the need to add the small explanatory tile that reads, “curiosidad antigua”.
At least the old Spanish villagers had the sense to come out and specify what sort of language is not acceptable. This listing (from the Motion Picture Association of America) isn’t exactly informative:

Of course, other films are rated PG simply because of “mild language” which seems even less reasonable.