One of the more common questions I hear: What language should I take with Linguistics this level? There are tons of languages on Golarion, but only a few are really worth learning, as most everyone speaks Taldane (Common) for some reason. You either want languages that you frequently find written materials of, or those that are spoken by creatures that actually don’t speak Common. I usually separate them into small categories. Here are, in my opinion, the most valuable languages to know as a Pathfinder.
Dead languages: Ancient Osiriani, Ancient Thassilonian, Ancient Azlanti, Aklo, and Jistkin.
Virtually no one speaks these, but adventures often have writings or wall carvings that can be deciphered by those who have learned them through Linguistics.
Elemental languages: Ignan, Terran, Aquan, Auran. These are important for speaking to elementals or other elemental planar residents. Aquan has the side benefit of being intelligible underwater, which can be useful on occasion for spellcasters and the like.
‘Alignment’ planar languages: Celestial, Abyssal, Infernal. These are mostly for writings, as most outsiders that the PCs could actually talk to speak Common. Writings/inscriptions in these languages are quite common. I would say Infernal comes up most frequently, and Devils are more likely to be reasoned with than Demons. Protean also exists, but virtually never comes up.
Humanoid languages: Dwarven, Elven, Tian, Iobarian. Writings in ancient forms of these languages occasionally come up, and some Tian NPCs actually don’t speak Taldane. Tian is probably the most important of this group, though Ancient Dwarven is something I’ve seen a fair amount in one particular season.
Others: Undercommon, Sylvan, Goblin, and Draconic. These mostly come up when dealing with certain troublesome NPC types, like Kobolds, or various fey. Sometimes those creatures don’t speak common. Of these three, I would say Sylvan is most important, as numerous sentient plants and other oddball critters are Sylvan-only speakers.