訳:They are not your pants.
The Japanese is incorrect It should be "それらは" "It is not your pants" does not make sense in English so this is a mistake
No, in fact the Japanese is correct. Unlike English, where we count a pair of jeans, pair of glasses or pair of scissors in the plural, they are always treated as a single object.
And even if there were multiple pair of jeans, it is absolutely correct to say "それは" (though それらは might be more natural) because there's no strict distinction between singular and plural numbers in Japanese.