I'm not trying to discourage you--honest. But, I asked the same question back in the early 3.0 days. I felt that, for example, a professor of English literature would have more ranks in a language-related skill than, say, a middle school English teacher, who might have a rank or two more than a typical common person. But, as everyone pointed out, and has since turned out to be true--the game very rarely moves in this direction, and skill points spent on such things are taken away from other things.
Unless you're playing a variant of the game where it really does matter that your character can tell the difference between a sentence of Shakespeare and a sentence of Milton, then it isn't worth the effort to create such a system, because your players aren't going to partake in it, and the effort will have been wasted.
That said, if you're creating a very specific campaign where such intricasies matter, then I can see the point. In that case, all you need to do, I think, is to have Speak Language have Int as the key modifier, and to assign DCs to tasks (for example, "Know author of a well-known passage: DC 20," "Know author of a rarely-known passage: DC 25," "Know author of an obscure reference: DC 30," etc.
You can include Read/Write Language as a separate skill, also with Int as the key modifier. The two should be tied together with a synergy bonus (i.e. 5 ranks = +2 in the other skill's checks) on a language-by-language basis. You'll then want some feats that allow for things like, if you have 5 ranks in 4 different languages, you can always make an Intelligence check to see if you can figure out the gist of something spoken in another language, etc.
Dave