"Mon pantalon est anglais."
This exercise is meant to stress that the French for "pants/trousers" is a singular word: "un pantalon".
Which does not prevent from using it in plural, if you have more than one pair of pants and provided you change the verb conjugation: "mes pantalons sont anglais"
ये पतलून इंगलिश्तानी (my pants are English)
from मेरा जूता है जापानी, one of the most famous Hindi songs of all time (http://en.wikipedia.org/wiki/Mera_Joota_Hai_Japani)