
That said, using another language should give more xp

You can have RP reasons to learn other languages, IE if you really want Illuskan over Thorass for example your character could be descendent from illuskan people. Keep in mind that DM's rarely give you an addition language, you have to plan your bonus languages accordingly.Thorass - Thorass is an old human language, the direct ancestor of Common. It was a pidgin variant of Chondathan that western Faerûnian traders developed to communicate with their counterparts in other lands. An inscription in Thorass will translate directly into Common, though it will often have a stilted and archaic form of phrasing and vocabulary.
Lands of Intrigue: Amn, page 4 wrote:Languages
As a merchant nation, Amn hears every tongue of the
Realms (and some other worlds and planes) in its markets. It
rarely takes more than an hour in Amn to find someone who
can translate any language spoken in the past few centuries
for a fee, of course. Still, merchants doing business in Amn
must brush up on the ancient trade tongue of Thorass, long
abandoned elsewhere for todays common language. All doc-
uments, contracts, court proceedings, and official scripts and
speeches used by Amnian merchant houses or the govern-
ment use Thorass. While an understanding of many lan-
guages is certainly helpful, the further one goes in Amnian
politics or trade, the less one needs any tongue other than
Thorass.