That is, when talking about a boxer. In tennis, it would be "gaucher".

The term IS used in boxing, fencing, football (left-footed) and other sports.

Exemple:
Aux États-Unis, certains magasins pour gauchers se sont donné un nom percutant: "South Paw", le redoutable crochet du boxeur gaucher...
--"Gauchers, ne soyez plus contrariés", éditions Hachette, collection Pratique, 1997.

My Oxford dictionary lists "southpaw" as meaning "left-handed", especially in boxing.

Further research brought up the site below, which deals with boxing and gives:

Fausse garde ; gaucher southpaw

http://crete.argyro.net/lexique/liste4.cfm?rubrique=BOXE

You will find many more boxing terms on this site, which is worth looking at if you are translating anything in that field.

Voici une fiche de l'OLF sur le sujet :

Domaine(s) : sport/boxe

left-hander

Terme(s) apparenté(s)
left-handed fencer
left-handed
left-footed player
southpaw

gaucher n. m.

[Office de la langue française, 1986]


Bonne chance!

Marc Dionne