One of the cerebellum's main functions is being responsible for automatic responses, such as related to movement (reflexes) but also has language functions. The functions related to language are similar to muscle reflexes-- the quick automatic responses you don't have to think about, such as when someone says "how's it going?" you say, "fine" without thinking about your response. I'm willing to bet you don't stutter on those quick everyday responses. I'm not a neuro or language expert, but I do have a bachelor's degree in speech-language pathology and I'm willing to bet the tumor is not causing your stuttering. I've never heard of any research showing links there. The rest of your language and verbal processing is done in your temporal lobe, in Broca's and Wernicke's areas, responsible for language comprehension and production. Those are far from the cerebellum.