Yes and no. It stays the same (as some other foreign words) if you follow the rules of classic ortograph. It takes the plural mark (an additional 's') if you follow the prescriptions of the ortographic reform of 1990.
Yes. Madam is singular, the plural form is "mesdames." It's actually a French word that we borrowed. Many words in the English language are borrowed from other languages.
Six-- French Violet-- French Comment-- French
Some English words borrowed from French include "ballet," "champagne," "garage," and "fiancé."
Only words borrowed from other languages, such as Wombat.
An example of borrowed words would be "sushi" from Japanese, "croissant" from French, and "pyjamas" from Hindi. These words have been borrowed into English due to cultural exchanges and adoption of foreign concepts.
Madame IS a French word. It needs no translation when using it in French. Many "English" words are actually borrowed from other languages, the word madame is one example.
Mots
The French use the words "un/une baby-sitter" as borrowed from English.
parking, email, weekend, job, hit-parade, and flirter, that the English borrowed from the French 'fleurette' (small flower) - (another list in link)
Some French words that start with "W" are "week-end" (weekend), "wagon" (wagon), and "wifi" (wifi).
Un bed and breakfast (these are borrowed words)
there is no plural for "en", even when all the other words are plural in the phrase: je suis en chemise - I am in shirt. ils sont en chaussures - they are in shoes