This unit is making me realise how many clothing words in English are always plural.
Why the esperanto sentence is singular and the english one is plural?
English treats certain things as plural, by considering them as two matched items joined in some way:
(A pair of) trousers (A pair of) scissors (A pair of) compasses (A pair of) pyjamas
But Esperanto treats them as a singular item (quite sensibly, as half-a-pair of trousers isn't much use to most people!)
Trousers - Pantalono Scissors - Tondilo Compasses - Cirkelo Pyjamas - Piĵamo
It's just a matter of how the language looks at those items.
pajamas is normally plural in English (the singular is used in some special cases (as an attribute and not as an independent noun) only (pajama-clad or a pajama party for example). Esperanto (like most other languages) uses the word both in plural and in singular - both versions will be translated the same in English (unless it is clear that a plural is needed and then the English will need to qualify in a different way).
Think of trousers - you have the same kind of thing (the singular is only as an attribute (with a special usage in the fashion industry).
I was just thinking that, as a native English speaker, I need to practice not pluralizing pijxamo and pantalono. As I tell students when I'm tutoring, "English is weird."