@julien.villiger you are correct in your understanding of Offline Speech to Text providers! Any application can register as one.. including your own! Magic Leap does not offer an online/offline Speech Recognition Provider as one of our offerings so if I was pursuing it in my own application I would consider an open source or paid provider. This is not an official recommendation but you can try experiment with using the Vosk offline provider if your use case is simple enough. Once you have a valid app registered as a provider these calls "should" work in Unity but if you are experiencing issues please reach out to Sidney again.
Good Luck,
Mark