If the command is in auto-translate, yes it will work.
Otherwise, it is not clear as not eveything is in the auto-translate: in particular, arguments of commands. For instance, the command to display hotbar 3 is /hotbar display 3. If /hotbar is in auto-translate, then on the French client you will see (/barracc) display 3 and it will work (I tested personnaly a few years back). Although the real French command is (/barracc) afficher 3. In general, /? will tell you if an English argument is valid on the French client.

If multiples commands yield the same result then usually one of the English one is imported in the French client. For instance, /pet will not work if not in auto-translate but /stroke will. Furthermore, if you type /stroke and then tab, you will see that il has been changed in auto-translate to (/caresser), while it will not work for /pet + tab. That allowed us to keep our FFXI reflex reactions by typing English emotes as /wave, /slap, etc. :-D.