Some people learning English would get confused as to why we say 'trousers' or 'jeans' etc when referring to a singular item. 'The trousers that I am wearing' to them they might think I have 2 pairs on. But it just takes learning the grammar of another language. Practice, practice, practice :-)
The reason "pants", "trousers", and "jeans" are all plural is that they have been shortened from "a pair of pants", "a pair of trousers", and "a pair of jeans".
Once upon a time they were just two tubes of cloth, one per leg, and attached separately at the waist. Thereof "a pair". Only later did someone think to sew them together but it was too late. They were already known as "a pair of" and it stuck.
Then later "a pair of" started to be omitted in casual language but the plurality remained. So "pants", "trousers", and "jeans".
Hope this helps.
Jeans in English is always plural: one piece for each leg - a pair of jeans, even when you talk about one item of clothing.
In french this garment is not considered to be a pair of jeans but one garment, thus in french it is singular: jean.
When you talk about two or more jeans in french you use the plural: jeans.
Basicly you never use "jean" in english since 1 jean is still considered plural and thus, called "jeans", as a commentor said above
-ENGLISH: "Jeans", the clothing is a pair(legs most likely), always plural
-FRENCH: "Jean", The French consider the clothing one piece, so the a single pair of jeans is "jean", however if you have two or more pairs of jeans, the french would say "jeans"
Hope you can understand what i'm saying, my explaining skills aren't that good and i'm on mobile