questions de francais

Well, that is slang alright! I wil translate the ones I know, leave it to French French slang speakers to translate the rest ....

la voiture accélère pied au plancher : acceleration to the maximum (when the speed pedal is down to the car's floor)

ça fait mauvais genre (depending of the context): gives the impression of being undereducated, lacking class, or behaves like s/one from the opposite sex

un sous-fifre : s/one in an inferior position, not in a position to take decisions

des mecs sur leur 31: guys dressed their best

"tant qu'à faire" : while we're at it (I'm not satisfied with this one - can't find more appropriate)

"sur leur 31" -- why 31? Same question for "dressed to the nines": why 9?

La voiture accélère pied au plancher: Saskja is right, but this does sound strange since it's not the car that has its foot on the gas pedal.

gniard: kids ?

ça fait mauvais genre: the first 2 meanings I agree with, wouldn't use the one about the opposite sex

ils sont pas prêteurs on le sait: literally, it is known that they don't like to lend their things (or money?)

le traité de Sevres morcellait ... The Treaty of Sevres broke Anatolia up into a Turkish rump state

pantins d'Irak nord - puppets of northern Iraq (s/b Irak du nord)

tant qu'à faire: I agree with Saskja here

poireautage: poireauter is tohang around waiting for something or somebody

As for the 31, I just looked and nobody knows the exact reason -- one hypothesis is that it comes from a fancy cloth called "trentaine" , another that it's for getting dressed up for Dec 31st. (less plausible in my opinion because it's an old expression).
As an added comment, the French department I live in is #31 and there is a jazz festival here called "Jazz sur son 31".

"Cette branlée" is "this beating". The text probably refers to a shameful, less than honourable defeat, does it?
"Gniard" is "kid, brat"
"On repart donc sur la vieille scie": "here we go again..."
" Apoist" is obviously mispelled..."Pur seker (sucre? "is "at its best", "at its purest".

grand merci.
est-ce que le g en gniard se prononce?
et
avoir le nez creux??

g before n makes a sound like n with a tilde in Spanish, is not pronounced separately.

Avoir le nez creux means to be good at finding things, like bargains.

branler and all its derivatives always follow a sense of shakeup and motion, in both physical and figurative way.... and of course, it always brings me a smile for one famous spanish connotation :)

pantin, fantoche pour l'espece de fendant. marionette pour les mortels comme nous.