Ceilliau, Bryn?
I'm still a learner, but even I can see there are clear differences between Cymraeg llafar and Cymraeg llenyddol. And yes, in every day use you write as you speak, but even then I'd expect to hear shwmai but read sut mae or even sut rydych chi.
And being a lawyer isn't really relevant as - unfurtunately - the language of the law in Wales is still English. I know that there is a right to use Welsh in proceedings, but I know of no court that doesn't conduct all its affairs in English.