For now, AI Speakers are only available in English.
Audio created using an AI Speaker or stock voice works differently than recorded audio in your script. You can't adjust the word boundaries or use the Timeline tool AI voice clips. Converting it to audio will convert your Overdub into an audio layer that you can edit like you would audio in the Timeline.
To convert an AI voice clip, hover over voice clip in the Timeline and select convert to audio.
If you're not seeing the Timeline, click Show timeline in the bottom left of the editor.