Okay, here's some brutal feedback, please take it with your best interests at heart. I am an English native speaker who has lived in Spain for 10 years, and has become fluent.
1) These speech-to-text models are poor when it comes to non-natives. This is unfortunate as the idea you had and the product you've designed could be incredible for language learning. However - it's a bit crap - sorry - I can speak Spanish well and was asked in the conversation if I wanted a medium sized cup of coffee, I replied "sí, mediano", the resulting text was outputted as "mariano", then in the role play the coffee shop worker then assumed my name was Mariano! Completely ludicrous and frustrating.... in real life the coffee shop worker is clearing expecting the word 'mediano' and will hear what I said and know that's what I was trying to say. The speech-to-text-model completely fails to get this.
Until speech-to-text models trained on non-natives are made readily available, products like this with so much promise will infuriate learners, which will stop them paying for it.
And this was ordering a coffee.... imagine an actually complicated conversation.
So my advice would be, right now the speech-to-text models aren't capable of doing what you're hoping they can do... but.... once you get a model that can, this will be insanely popular....
So hang in there, other than that it was a fun experience, and critically, people are scared of practising with real people, something like this would be insanely popular if it actually worked well. Good luck.