Download your phone's offline speech recognition for both your languages — the one you speak and the one you are learning.
With a connection, your phone's speech service hears your answers online. Without one, the phone can use only the speech models stored on it, and it has them only for the languages you have downloaded. Do it once, while you are online.
On Android: open Settings and search for «Android System Intelligence». Open it, then On-device recognition › Add a language, and add both languages. These names are from a Samsung Galaxy; other phones may name them differently. Languages downloaded for offline translation don't count — speech recognition needs its own download.
On iPhone: Settings › General › Keyboard › Dictation Languages — add both languages.
If a language is still missing, the app tells you which one before the lesson starts.
