Other forms: locutions
Your southern-born friend's habit of saying "y'all" when she's talking to her family could be described as locution — it's a word she habitually uses in particular situations.
A person's style of speech — certain words or phrases she tends to use — is her locution, especially if it's specific to a particular place or group of people, like teenagers or French Canadians. You can also use the noun locution when you're talking about the way a person pronounces words. If someone tends to "beat around the bush," or tell a story in a roundabout way, you can call that circumlocution.