Corresponding loanwords in two languages are difficult to find. One example of adjacent loanwords is the following:
French: le pull[-over], le bikini
So the English maillot can refer to a few garments, as Merriam-Webster indicates. Two of the garments are a swimsuit (usually a one-piece women's suit) and a jersey. Most image or product searches for maillot yield a swimsuit, as seen in these Google image results:
At least historically, maillot could also refer to a jersey, as some Oxford English Dictionary quotes attest:
1948 At a gala evening where the men would discard their maillots for starched linen. A. Waugh, Unclouded Summer iv. 65
1948 His heart was thudding against the cotton of his maillot. A. Waugh, Unclouded Summer iv. 78,
1955 In a not quite clean maillot and a seersucker skirt. D. Barton, Glorious Life vi. 74
French has at least two non-exact loanwords from English.
The French le bikini is derived from the Anglo-American Bikini (itself from a German name for the atoll, in turn from the Marshallese Pikinni [Wikipedia, Bikini Atoll]), which was the name of the atoll where the US Navy carried out atomic bomb tests. Fashion designer Louis Reard borrowed the word from news reports of the first atomic tests, and the term then quickly spread across the fashion world. In a way, bikini in English is thus a double loan-word, lent to French and quickly taken back into English in a different sense.
The term is not quite synonymous with the swimsuit maillot. A maillot is almost always one-piece. A bikini is two piece.
As for le pull or le pullover, it refers specifically to something like this:
It's hard to say whether this compares to the once-used jersey sense of maillot. Two possibilities are that the maillot was really a traditional jersey, in which case it's very close to le pull:
and that it was closer to a modern sports jersey, which would make it not so close: