All humans in Arvum speak a common language developed long before the Reckoning known as Arvani, Compact Common or some other variation. Given the general lack of foreign contact, it is rare for citizens of the Compact to be bilingual and is only not uncommon amongst scholars, adventurers, and the idle rich desperate for a party trick. Eurusi and Cardian sailors have been heard speaking their own languages during their rare port calls but it is completely unheard of for them to teach it.
The Abandoned of each region have developed their own dialects, but scholars, adventurers and the idle rich who stumbled into an awkward ransom situation while accepting what they believed to be a dinner invitation all agree that these dialects are derivatives of the common tongue. Virtually all Abandoned speak Arvani as their main language and the dialects are primarily for use around hostile (i.e, the Compact) groups.
Please don't use real-world languages as flavor substitutes without asking first. Some are already used for that purpose and it could give the impression that your character knows languages they would not.
Each level of Linguistics increases your language slots in the +lang command by the following amount:
Level 1: +1 slot
Level 2: +1 slot
Level 3: +2 slots
Level 4: +2 slots
Level 5: +3 slots