#back now!

Mmm, I don't know for sure, but I'd say that it was probably technically French? Auvergnat was still (still is!) spoken in the region, but it wasn't at its height as the dominant language any more (that was in the 1500s), and I think being from an aristocratic family and his father canonically being so conscious of class and how the family is perceived by other nobility, he would've wanted all of his children to speak French.

I also think it comes down to how he learnt to talk, which was probably through listening to his mother, and I think it's probably pretty likely that Gabrielle wouldn't know much Auvergnat? She's Italian, so that's her mother tongue (and she I'd imagine she spoke that quite a bit to Lestat), but she was educated by her family and I'd say probably taught French as a child, particularly if her parents were intending to try and marry her into foreign nobility. She wouldn't have been taught Auvergnat, and her isolation with the Marquis in the castle probably gave her little avenues to learn it after they were married.

That said, we also know Lestat had a nurse for a little while when he was very small, and he probably also learnt how to speak from listening to her and there's a good chance that she spoke Auvergnat?