I've always hated the idea of a "common" tongue and so I've done away with it in my homebrew. I've also adjusted the Speak Language skill as well, big thanks to a fellow player that came up with this. Admittedly I haven't playtested this yet, that's coming in the next campaign, but it seems to be sound mechanics.
1) At 1st level all characters begin with 5 ranks in one Knowledge ("language") skill, their native tongue. These free ranks are not restricted by skill cap, though additional ranks are still restricted by skill caps.
2) Each additional language is treated as a Knowledge skill, and thus you can spend as few or many skill points on each language. At 1st level you will get 1 + Int bonus in skill points to be spent solely on the Knowledge ("language") skill, skill cap rules still apply. The Knowledge ("language") is considered a class skill only for those classes that had Speak Language as a class skill. After the initial free skill points at 1st level, PCs must spend skill points on Knowledge ("language") just like any other skill. Just because you can speak/read a language, doesn't mean you have perfect mastery of it.
3) Fluency is 5 ranks in Knowledge ("language").
4) If you attempt to use any interaction skills (bluff not used to feint, intimidate not used for combat applications, etc.) you recieve a -2 language penalty.
5) Fluency in a language (5 ranks) gives a +2 synergy bonus to interaction skills undertaken in that language, offsetting the above penalty. Note that native speakers (who begin with 5 free ranks) effectively offset this penalty immediately.
6) 10 ranks in a language gives a second +2 synergy bonus to interaction skills. A character's mastery of this language is so sublime that they are able to get more out of people than they even realize.
Languages of Vetus
Abyssal
Aquan
Auran
Celestial
Draconic
Druidic
Dwarven
Elven
Giant
Gnoll
Halfling
Ignan
Infernal
Sylvan
Terran
Pelyrene - The native language of humans and gnomes.
Illari - This ancient language, said to be the tongue of the mythic Illarium Empire, is the language used by the clergy of Claris Sol . It is also the language used in many scholarly texts of the Pelyrene Empire .
Grusch - The native language of hobgoblins, goblins, ogres, bugbears and orcs.