In French, a single piece of clothe is singular.

For instance a sweater would be un pull over or un sweat. Shorts would be un short, and pants/trousers would be un pantalon.

Un jean being one specific kind of pantalon, would be singular too.

Pantalon obsoleted chausses around the French revolution. Chausses (which gave the modern chaussettes – socks) itself obsoleted braies (related to the braguette – trouser fly). Both braies and chausses were used in plural when naming the two legs garment.

When pantalon was introduced, from the Commedia dell’arte character’s Pantalone/Pantaloon who was wearing red pants, plural and singular were competing for a while but the singular seems to have won in the 19th century, probably because pantalon dual nature was lost or rejected.

Here is an excerpt from the Nouvelle orthologie française, 1832:
 
           

The plural (une paire de pantalons) survived as a regionalism in Belgium and Switzerland and even more in Canada where it seems to be still the norm but has almost disappeared entirely in France although it might survive with elderly people in Southern France, Brittany and other regions.

A Ngram comparison of the percentage of singular vs total occurrences of a sample of words clearly shows pantalon follows almost the same usage pattern than chemise since at least 1880 so is clearly not in the same group than dual words like gants, ciseaux or chaussettes. Of course this is only based on books scanned by Google so might not perfectly reflect the spoken French reality, as Laure’s personal experience might suggest.

It is worth noting that this competition between singular and plural exists in Italy where un paio di pantaloni (“a pair of”+plural) is the traditional and widespread name, but un pantalone (singular) is often used in shops.

In France, jean is following the same pattern than pantalon so is always used as a singular (un) although sometimes the final s is kept (un jeans).

There is the same difference with short (French: un short, English: a pair of shorts).

Several similar “dual objects” are named using the plural in French, e.g.: ciseaux, bretelles, lunettes, jumelles…

The dual nature of these words is not questionable because they are made of two parts which have a individual name (un ciseau, une bretelle, une lunette …) while pantalon (from a single person name) or jeans (from a town’s name) are not etymologically components to be assembled in pairs.

Almost the opposite happened with “pin” which was adopted in French as un pin’s.

Both are used un jean or des jeans where one means a single pair of jeans. People will say un jean where they say un pantalon for a pair of trousers. We can add culotte and short to the list.

In France people mostly use these words in the singular when talking about a single piece of clothing and it is how dictionaries say the word is used nowadays, the plural for a single piece of clothing is noted as outdated in writing, but it was still used in the plural in writing until the early 20th c. as either pantalons or une paire de pantalons.

C’est un gaillard qui en dégoise ; il porte une barbe longue d’une aune, des pantalons collants, un habit à larges revers, et un bolivar sur la tête,… (Alfred de Musset, Lettres de Dupuis et Cotonet au directeur de la Revue des Deux-Mondes, 1836).

Some people still use the plural form for a single piece of clothing in the south of France, because of the influence of Occitan where the plural is used for a single pair of trousers.

Concerning Québec we find this testimony from a Quebecer:

Au Québec et au Canada français, comme il arrive souvent, les usages anciens ont mieux résisté à l’érosion du temps. Même si l’expression paire de pantalons a pris de la bouteille – je l’utilisais couramment dans ma lointaine jeunesse -, le pluriel pantalons désignant un seul vêtement reste encore d’usage courant. Le mot culottes, pour sa part, est demeuré très fréquent tant avec paire que sans. (Pantalons, culottes et jeans : pluriels énigmatiques)

According to the wiktionary In Belgium and in Switzerland the plural can still be heard.

Pantalon is a rather recent word (17th c.) derived from the name of the character in commedia dell’arte. We can presume the singular established itself in the end because of the proper name, but as well because what used to be a two part piece of clothing is now seen as one single piece. Before pantalon appeared French used words in the plural to designate a piece of clothing that covered entirely or partially both legs at the same time : braies (from Latin braca for one trouser leg), chausses and caleçon both deriving from Latin calceus that also gave chaussure and chausson.

Plural to singular can also been explained by the change in fashions. Here an extract from a thread on Projet Babel that of course would need to be substantiated by historians but that I am inclined to believe:

J’ai aussi entendu indistinctement des pantalons, une paire de pantalons, ou un pantalon.
Il se trouve que les pantalons des dames étaient faits de deux jambes rigoureusement indépendantes l’une de l’autre : seule les solidarisait la coulisse de la taille. Ce dispositif permettait de communier avec la nature sans avoir à baisser le dit pantalon, dont il fallait retrouver la coulisse sous trois épaisseurs de jupon…
Les danseuses de cancan avaient ordre de coudre l’entrejambe de leur pantalon, et les spectateurs, bien entendu, n’attendaient qu’une chose : que la couture craque…

A la même époque, les culottes des petits garçons n’étaient pas non plus cousues, pour le même motif : leur permettre de se soulager sans avoir à se battre avec les boutons. Il arrivait qu’un pan de chemise passe par la fente : observez attentivement les gravures qui illustrent les œuvres de la comtesse de Ségur, vous verrez ce pan de chemise des petits bonshommes de trois quatre ans…

Donc, deux jambes indépendantes font une paire de pantalons, mais maintenant que tous les pantalons ont un fond solide, il n’existe plus que le pantalon singulier.

To be read also: De singuliers pluriels.

L’usage peut varier selon les régions et le temps, comme on en discute ailleurs. On peut cependant noter simplement que :

De nombreux noms qui désignent un objet composé de deux parties
identiques peuvent être employés tant au singulier qu’au pluriel. Dans
un cas, on considère l’objet ou le concept en soi; dans l’autre, on
considère les deux parties qui le composent.

[ Banque de dépannage linguistique (Québec), Noms pouvant être
employés au singulier et au pluriel ]

Si on utilisait indifféremment (contra Larousse en ligne) jean et jeans (de blue-jeans), l’emploi au singulier pourrait servir à marquer davantage l’objet dans son entier par opposition à ses parties constituantes qui sont identiques. Ce serait une manière d’expliquer un tel choix si l’on veut s’en donner un. Ainsi mon jean neuf, mon jeans neuf et mes jeans neufs pourraient tous signifier un seul jean(s), mais différemment. Le contexte et d’autres noms etc. peuvent servir à préciser s’il s’agit d’un seul ou de plusieurs objets.