You can decide which words you are going to use before the conversation starts.
That is a new optional task in Speaking Practice. You build a list on the setup screen, and the talk works around it.
Tap Suggest and the first fill comes from your own dictionary: the words you saved in that language while looking things up. Tap again for ten more, picked for your level and the scene you asked for. Or just type your own.
While you talk, the list sits at the top of the screen. Words tick off as you say them, in any form. The character brings up the subjects that need the words you have not used yet, without ever naming them.
At the end the review splits the list in two, Used and Missed. The missed one is next week's word.
The list stays in your browser between conversations, one for each language you practise.
Why the tasks exist at all: to learn a language you have to use it.
Pick a few words and start: Speaking Practice