One modification I would suggest is to add a bit more to the initial prompt like:

"write as if you are a person from {{REGION}}. Modify your language to proficiency level {{PROFICIENCY_LEVEL}}"

that way I could for example, speak as if it's someone using Mexican Spanish vs Madrid Spanish vs Chilean Spanish, etc.

Secondly, you could include the user's speech transcribed as part of the conversation window

Amazing idea, do you think this should be a freeform text field the user can enter to add their own prompts to or should it be a checkbox/select on the homepage so the user can pick from a limited set?