Actually, if you're going to stay local or if you have an idea about what your DM will throw at you, i'd suggest those languages. More commonly spoken languages can come more in handy then long forgotten languages unless you routinely discover ancient tombs or ruins from a long lost civilisation...which really shouldn't be routine no matter who you are...unless ofc you're a 20th level wizard...anyway, i'd suggest as Fax said elemental languages, maybe celestial or abyssal/infernal depending on alignment, the languages of your teammates...

If you're looking for a true linguist then you might go for changeling with the master linguist feat, i think most DMs would allow it though i don't think you're looking for this exactly...the solution would be to ask help from your DM, what kind of languages are most probable to encounter...if you have the FR campaign setting or the one you could go through it, but most grand old civilisations didn't build underground as much..firstly for splendor and secondly because they were confident in their power...though that would depend on your DM...

As suggested above, netherese is your best bet and ask your DM, he should've looked through the campaign setting and could propose a few languages...also since you have speak languages as a class skill you could spend a few skill points with your high int for extra languages...