not every planescape players and DMs have english for first language. Sure we have no problems of reading but we do our games in french (or spanish or german or whatever...). Because of this, it's not very possible to use the original english cant so i wonder if no one as ever try to translate it, if it's the case juste let me know !
the cant in french
The question has come up before and I don't remember there being a satisfactory answer to this. There was a project to translate planewalker.com in to Spanish and perhaps you could borrow some words if cant was transalted there. Is the Spanish PS site still around?
After a search I came up with the following suggestions. You could either use an older form of French such as: Langues d'oïl (see http://en.wikipedia.org/wiki/Langue_d%27o%C3%AFl).
Or you could go for a variant of French that is in use and from this page is used by those trying to steer clear of the law to hide their meaning. Here is one link with more information: http://french.about.com/library/vocab/bl-verlan.htm and here is another: http://en.wikipedia.org/wiki/Verlan.
I'm not very knowledgeable of French regretably and so I cannot help much further. Although I would be very interested to hear if either of those suggestions are at all useful?
Cant evolved in Elizabethan England (and most likely before that) for criminals to hide their meaning and hence it has evolved over time. There are phrases used today that are volutions or additions from past time. Wikipedia provides more information about cryptolects (language use to hide meaning) here: http://en.wikipedia.org/wiki/Cant_%28language%29.
Just to give you an example of modern cant: "'Ere, how's it with the strife? Getting any how's yer father then? Or is she going to be on the bone all night to her indoors? In that case - why splash a pony at the dogs and then have a ruby?"
Well, for the cant in french, since the basic box set have been translated in french, we use these. For more, we use "l'argot de paris". wich is a thief slang from 17th and 18th century. Here's the list we use:
Dictionnaire Argot voleur-Français
A jeûn = Vide
A la grive = Prend garde!
Abbaye de Monte-à-regret = Echafaud
Abouler = Venir
Affranchi = Libéré
Ardents = Yeux
Arpions = Pieds
Arquepincer = Prendre
Atout = Courage
Autor = Autorité
Avoir ses bringues = Saigner
Battre l'antif = Espionner
Béquillard = Bourreau
Bibarder = Vieillir
Birbasse = Vieille
Boulanger = Diable
Boulanger qui met les âmes au four = Diable
Buter = Tuer
Calotte = Baffe
Camarder = Mourir
Capahuter = Voler une part de butin
Carline = Mort
Charlot = Bourreau
Cheval = Repris de justice
Chiffon rouge = Langue
Chourineur = Assassin
Clou = Prison
Colas = Cou
Coloquinte = Tête
Coquer = Dénoncer
Coupe-sifflet = Couteau
Crible = Crier
Cuir de brouette = Bois
Curieux = Juges
Daims huppés = Gens riches
De retour = Arrêté une nouvelle fois
Défourailler = Courrir
D'esbrouffe = De force
Dévider = Parler
Donner un pont à faucher = Tendre un piège
Dur = Fer
Durailles d'orphelin = Pierreries
Eau d'aff = Eau-de-vie
Endêver = Enerver
Enflaquer = Perdre
Entailler = Tuer
Escarper = Tuer
Fagot = Forçat
Faire la tortue = Jeûner
Fauchants = Ciseaux
Faucher = Exécuter
Faucher dans le point = Tomber dans le piège
Ferlampier = Bandit
Fièvre cérèbrale = Condamnation à mort
Filoche = Bourse
Flotter = Noyer
Fourline = Assassin (diminutif de Fourloureux)
Fourloureux = Assassin
Frileux = Peureux
Gaye = Cheval
Gernaffle = Ferme
Gironde = Jolie femme
Gosseline = Fillette
Goualeuse = Chanteuse
Gouêpeur = Vagabond
Gras-double = Lames de plomb (généralement volées sur les toits)
Grinche = Voleur
Il va y avoir du raisiné par terre = Le sang va couler
Jars = Argot
Jaspiner = Causer
Largue = Femme mariée
Loup = Créancier
Macaroner = Trahir
Manger sur nous = Nous dénoncer
Manille = Anneau qui tient la chaîne des forçats
Mannequin = Corbillard
Marcher dessus = Manquer
Médecine = Conseil
Meg des megs = Dieu
Messières = Victimes
Milord = Carosse
Mirettes = Yeux
Mon linge est lavé = Je m'avoue vaincu
Muette = Conscience
Négresse = Caisse entourée de toile cirée noire
Nourrir = Organiser
Ogre = Repris de justice qui tient un tapis-franc
Orient = Or
Pante = Victime
Passe-lance = Bateau
Passésinge = Criminel habile
Perte de vue = Perpétuité
Pessiller = Prendre
Pitancher = Boire
Planche à pain = Guillotine
Plume de Beauce = Paille
Poupard = Vol
Pré = Galères
Prend-garde-à-tout = Surveillant
Profonde = Bourse
Quart d'oeil = Commissaire
Raille = Mouchard
Raisiné = Sang
Rat de prison = Avocat
Ravage = Débris métallique
Refroidir = Tuer
Riffauder = Chauffer
Ronds = Sous
Rouget = Cuivre
Rousse = Police
Sanglier = Prêtre
Se refaire de sorgue = Se nourrir
Serrer = Emprisonner
S'esbigner = Se dépêcher
Singe = Bourgeois
Sinve = Homme simple
Sondeur = Commis
Sorconner = Raisonner
Surin = Poignard
Tapis-franc = Cabaret mal famé
Teignasse = Coiffure
Tétards = Intellectuels
Tortillard = Boîteux
Traviole = Traverse
Trimballer = Transporter
Trimballeur = Cocher
Turf = Terrain de course où ont lieu des paris
Voite = Véhicule
Zig = Camarade
Dictionnaire Français-Argot voleur
Anneau qui tient la chaîne des forçats = Manille
Argot = Jars
Arrêté une nouvelle fois = De retour
Assassin = Chourineur
Assassin = Fourloureux
Assassin (diminutif de Fourloureux) = Fourline
Autorité = Autor
Avocat = Rat de prison
Baffe = Calotte
Bandit = Ferlampier
Bateau = Passe-lance
Boire = Pitancher
Bois = Cuir de brouette
Boîteux = Tortillard
Bourgeois = Singe
Bourreau = Béquillard
Bourreau = Charlot
Bourse = Filoche
Bourse = Profonde
Cabaret mal famé = Tapis-franc
Caisse entourée de toile cirée noire = Négresse
Camarade = Zig
Carosse = Milord
Causer = Jaspiner
Chanteuse = Goualeuse
Chauffer = Riffauder
Cheval = Gaye
Ciseaux = Fauchants
Cocher = Trimballeur
Coiffure = Teignasse
Commis = Sondeur
Commissaire = Quart d'oeil
Condamnation à mort = Fièvre cérèbrale
Conscience = Muette
Conseil = Médecine
Corbillard = Mannequin
Cou = Colas
Courage = Atout
Courrir = Défourailler
Couteau = Coupe-sifflet
Créancier = Loup
Crier = Crible
Criminel habile = Passésinge
Cuivre = Rouget
De force = D'esbrouffe
Débris métallique = Ravage
Dénoncer = Coquer
Diable = Boulanger
Diable = Boulanger qui met les âmes au four
Dieu = Meg des megs
Eau-de-vie = Eau d'aff
Echafaud = Abbaye de Monte-à-regret
Emprisonner = Serrer
Enerver = Endêver
Espionner = Battre l'antif
Exécuter = Faucher
Femme mariée = Largue
Fer = Dur
Ferme = Gernaffle
Fillette = Gosseline
Forçat = Fagot
Galères = Pré
Gens riches = Daims huppés
Guillotine = Planche à pain
Homme simple = Sinve
Intellectuels = Tétards
Je m'avoue vaincu = Mon linge est lavé
Jeûner = Faire la tortue
Jolie femme = Gironde
Juges = Curieux
Lames de plomb (généralement volées sur les toits) = Gras-double
Langue = Chiffon rouge
Le sang va couler = Il va y avoir du raisiné par terre
Libéré = Affranchi
Manquer = Marcher dessus
Mort = Carline
Mouchard = Raille
Mourir = Camarder
Nous dénoncer = Manger sur nous
Noyer = Flotter
Or = Orient
Organiser = Nourrir
Paille = Plume de Beauce
Parler = Dévider
Perdre = Enflaquer
Perpétuité = Perte de vue
Peureux = Frileux
Pieds = Arpions
Pierreries = Durailles d'orphelin
Poignard = Surin
Police = Rousse
Prend garde ! = A la grive
Prendre = Arquepincer
Prendre = Pessiller
Prêtre = Sanglier
Prison = Clou
Raisonner = Sorconner
Repris de justice = Cheval
Repris de justice qui tient un tapis-franc = Ogre
Saigner = Avoir ses bringues
Sang = Raisiné
Se dépêcher = S'esbigner
Se nourrir = Se refaire de sorgue
Sous = Ronds
Surveillant = Prend-garde-à-tout
Tendre un piège = Donner un pont à faucher
Terrain de course où ont lieu des paris = Turf
Tête = Coloquinte
Tomber dans le piège = Faucher dans le point
Trahir = Macaroner
Transporter = Trimballer
Traverse = Traviole
Tuer = Buter
Tuer = Entailler
Tuer = Escarper
Tuer = Refroidir
Vagabond = Gouêpeur
Véhicule = Voite
Venir = Abouler
Victime = Pante
Victimes = Messières
Vide = A jeûn
Vieille = Birbasse
Vieillir = Bibarder
Vol = Poupard
Voler une part de butin = Capahuter
Voleur = Grinche
Yeux = Ardents
Yeux = Mirettes
While I don't know of any translations, I can give you a few suggestions on how to start out:
Cant (as one of the Planescape supplements explained) is actually based on English slang used in London just before the Industrial Revolution. My advise is to adopt a version of French slang that has a similar cultural emphasis (practical and cynical, with just a hint of gallows humor).
Faction Nicknames are a little trickier. I can think of a few to start with (don't worry too much about proper grammer):
(Fated) Heartless -> les sans-coers
(Dustmen) Deaders -> les morts