I don't think Finnish (or any language in a state of diglossia) is the best example here. That is, Finnish is already essentially two separate languages. Written Finnish (kirjakieli) is a somewhat artificial language that was a compromise between different dialects, because at the birth of Finnish literacy there was no longer an "original Finnish" on which to base writing.
Written Finnish is mainly a written thing, and while there is a close correspondence between the orthography and how it would be read aloud, when Finns actually speak they use spoken Finnish (puhekieli), which isn't standardized and varies from region to region.
I am referring strictly to the correspondence between a written language and its pronunciation. The fact that people speak differently from how they read/write is an altogether separate matter.
When reading Finnish, one pronounces the words very closely to how the word is spelled, regardless of whether when they speak they do so in an altogether different manner.
Written Finnish is mainly a written thing, and while there is a close correspondence between the orthography and how it would be read aloud, when Finns actually speak they use spoken Finnish (puhekieli), which isn't standardized and varies from region to region.