Point of order:
This is the exact language of the charm:
The choice of words is significant. Becaue it suggests both reading and writing.
Inscriptions on ancient walls, hieroglyphics, that sort of thing.
This is narratively because its the Dresden Files.
Spirits speak human languages here, and only human languages, because they are largely a reflection of human society. Fae fashions reflect human society and changes in it.
Dr Miyamune, the baku from Day One, literally spawned from the NeverNever less than ten years before its showing up and only speaks English and I think Japanese. Ulsharavas the divinatory spirit speaks English. Chaunzaggoroth the demon speaks English. The Hecatean Hag spoke English and Greek. The Accords are in English. The spider-fae that Dresden was almost ambushed by on the road to Edinburgh in Turn Coat speak English.
All ghosts are spirits, and the sapient ones speak and write human languages.
This isnt like vanilla OWoD, where some spirits have a separate language, at least in Mage and Werewolf.
No spirits here do.
Besides maybe Angels.
Even in Exalted, Old Realm was the same language spoken by gods, spirits and humans in the Age of Glory.
Same language used for Exalted sorcery.
No differentiation.
It says 'the language of spirits' not all languages spoken by all spirits. Unlike WoD (by way of Werewolf the Apocalypse) this world does not have a universal spirit language as you said so you get whatever the native language of the spirit you are talking to at the time is. I get how how you reached that conclusion, but the way the difference between this world and WoD works out more towards being able to talk to spirits than learning all languages since that is the point of the charm by RaI, from what I can tell.