Slang is a collective term. It has no plural form.
It actually means this. squirrel or Squirrels for plural.
The only correct plural for French is French. "Frenches", as you say, or 'Frenchies' is a derogatory slang term.
The noun copper is an element. The adjective form means made of copper, or copper-colored.The noun "copper" meaning a copper coin (or slang for a policeman) would have the plural coppers.
Jolly is almost always used as an adjective. The related noun "jollies" is used in plural form and means festive times (UK) or slang for amusement (US) in the usage to get one's jollies.
The spelling "dudes" is the plural form of dude, slang for a guy or rural Western slang for someone from the East, or similarly dressed.
The plural of the French noun dico is dicos(slang for dictionaries, dictionnaires).
Player is used as a slang word, but if you want to slang it even more I guess it would be "playa." Plural = playaz
The plural of the slang term "hickey" is hickeys.
The plural of you is you. (The form youse is slang.)The word with that pronunciation is the plural ewes(female sheep).
Brains is both a plural usage of the word, and slang terminology to imply hitting someone about the head.
The plural of nephew is nephews, and the plural of niece is nieces.The rarely-used slang term that includes both male and female is nibling, plural niblings.
juice
The slang term "ex" (as in ex-spouse) forms the plural "exes", although neither is formal English.
The slang term for an emoticon is spelled smiley(plural smileys).:-)
In Brooklyn slang, the plural of "you" is "youse." Is is pronounced the same as "use."
The word 'phenom' is a slang or informal form for the noun, short for phenomenon (the plural for is phenomena, an irregular plural).Since the use of the word 'phenom' is relatively modern, the plural form can be a regular plural: phenoms.
Yes. The plural of "dude" is "dudes" (slang for guys, males, buddies).