Hmm…there may be a purposeful intention with the way languages are set up, in the game itself. But honestly, this is something that may need your own determination for your game specifically.
For ancient language, you could determine that the language could be read only (and by extension written). Because it’s an old language, there may not be a known pronunciation. This makes Ancient Language akin to reading, but only for that old language.
To a certain extent you should also determine, in your game fiction, the validity of the read and write skills. In Medieval games (the default), speaking is standard,reading is rare, and writing even rarer. They really were three different abilities (this accounts for why even fluent and learned persons, such as Shakespeare or Chaucer, had massive grammatical and spelling errors by current standards…the language wasn’t perfectly codified until very recently in history, and medieval europe was a mess as far as literacy goes…).
But in your game, perhaps these skills are moot…if you can speak it, you can read it and write it.
Now, for foreign languages, I can attest to the way this could work based on the 3 skill requirement. I don’t have anything near to fluency, but for the language I’m closest to, I can read it better than I can speak it, which is better than I can write it. My proficiency is variable for each area, meaning I can see how this works.
What I would do, however (to save yourself book keeping headaches for languages), is roll for the language you want to read, write, or speak (besides your native tongue), and FoRK in reading or writing, depending on which action you want to do. This advances the language, reflecting your increasing proficiency with it, while keeping your reading or writing skills static. This actually makes sense, because (again, from personal experience) the proficiency in a language, when read or written, is affected by your spoken proficiency and proficiency in standard reading or writing.
A good way of understanding this is looking at Grimm’s law (they were also linguists). Being proficient in the rules of grammer in your native tongue CAN translate to insight into grammar rules for other languages, especially if you have some passing knowledge of speaking the other language. In this case, me understanding that F in father (from English, a Germanic Language) is roughly approximated into P for Padre (from Spanish, a Latin language), means that for Germanic languages I can expect an F in those cases where-as for Latin languages I can expect a P.
Give me a B5 in reading, a B1 in Spanish, and that pretty closely approximates how I am much more proficient in reading Spanish than I am at simply speaking it. I FoRK all the time. And in my reality/fiction for the real world, reading and writing are synonymous skills.
Hope that helps (in a lengthy, overly detailed way)…