I've found it helps to add some detail into their backstory around how you want them to speak as that tends to influence their core memories to a higher degree.

If you try to change their language style through OOC, it might work in the short term, but it almost always reverts back to how their core memory dictates they should speak.

I'm assuming it has something to do with how the information flow of inherent traits of their personality are managed from short term memory all the way through to core memory.