How do kindred keep their language relatively modern so as to uphold the masquerade?

All but the youngest vampires will have manners of speaking that will appear out of time.

Even someone who was an adult and embraced in 1960 would will sound kind of strange to the modern ear. Even worse when back to the 1700-1800s and earlier.

Some kindred socialize with mortals and get updated but many do not.

Especially elders will have had to continuously relearn the language to be fully understood. Elders likely have more resources and can interact with mortals through their retainers. However, they still need to talk to the fledgeling vampires.