Here are some ways to translate it depending on the context:
1. "Modryb Mattie"
* Modryb means "aunt" in Welsh, but it's a more formal term. This would be appropriate if you were talking about an aunt who is older or you don't know well.
2. "Mami Mattie"
* Mami is a more informal and affectionate term for "aunt." This would be suitable if you're referring to a close aunt, perhaps one you've always called "Aunty Mattie."
3. "Mattie"
* If "Aunty Mattie" is a nickname or a term of endearment, you could simply use "Mattie" in Welsh.
4. "Auntie Mattie"
* In some cases, people might choose to keep the English term "Auntie Mattie" as it's a familiar name.
Ultimately, the best way to translate "Aunty Mattie" depends on your relationship with the person and the context of the conversation.