Just being a lizard with pet pal apparently doesn't qualify for this "Ancient Language." There seems to be some other pre-requisite.

Ok i got it , i took the Red Prince and take the Pet Pal Ability for him and he could understand the lizard
If you don't have the Red Prince in your party , try a lizard with pet pal and scholar.

For talk with lizard you need a lizard scolar in party. If you dont have one Fain can turn to with his mask.

I managed to open this chest without a lizard in my party at all... I don't quite remember how I did it, but I must've spoken to the salamander with my elf (pet pal) and then teleported the chest out and chose the right option. I opened the chest without a lizard AT ALL

I... broke it..

If you have taken fane in your party you will have a mask that can transform anyone into a lizzard. As long as you have someone with pet pal, use the mask, make him a lizzard, and the salamander will speak to you.

Just to make clear, you NEED to move the chest out of the flames in order to speak the password, since you can't while it's burning, even if you already got the pass.

If you have high persuasion you can just convince the chest to open up for you

thrice bound***** wish i had just checked a stupid guide as lockpicking ther damn thing loses you 20k exp as it does not talk! if you lock pick the stupid thing