I've been playing with variant language rules, suing a "tree" of related languages. I'll try and attach an .rtf file of a partial version of the tree I use. The basic premise is that the first languages were Celestial and Draconic, and most all other languages decended from them. Most languages are very closely related to each other, and so they're pretty easy to learn.
Basically, the rules work like this: everyone is fluent in their native language. They can make themselves understood in any language that's one "jump" away from their native language, but can't express complex ideas. Taking ranks in any language that is one "jump" away from your native language is considered a class skill for you, and fluency can be bought with one rank. Any other language is a cross-class skill, and buying ranks in that language gives you basic conversation, as if it was related to your native language. One more rank gives you full fluency, and basic conversation in all languages one "jump" away from the new language.
This system makes it easy to learn dozens of languages, since most of them are so closely related. It's a little more plausible than a "common language" that everyone happens to speak, but it has about the same effect: it's not hard to talk to other characters in the game. I can still throw in an oddball language (beholder-ish is not particularly related to anything else, for example) that no one can speak if I want language barrier issues to factor into an encounter, but it's easy to talk to most commonly-encountered people, and I find the game moves along better that way.
Basically, the rules work like this: everyone is fluent in their native language. They can make themselves understood in any language that's one "jump" away from their native language, but can't express complex ideas. Taking ranks in any language that is one "jump" away from your native language is considered a class skill for you, and fluency can be bought with one rank. Any other language is a cross-class skill, and buying ranks in that language gives you basic conversation, as if it was related to your native language. One more rank gives you full fluency, and basic conversation in all languages one "jump" away from the new language.
This system makes it easy to learn dozens of languages, since most of them are so closely related. It's a little more plausible than a "common language" that everyone happens to speak, but it has about the same effect: it's not hard to talk to other characters in the game. I can still throw in an oddball language (beholder-ish is not particularly related to anything else, for example) that no one can speak if I want language barrier issues to factor into an encounter, but it's easy to talk to most commonly-encountered people, and I find the game moves along better that way.