Make the preference explicit
Please answer in French using short, everyday sentences. Keep Mara's name unchanged. Ask one question, then wait for my answer.
If the conversation has used another language, repeat the preference alongside your next question. It can take several exchanges for a changed preference to become consistent. Avoid mixing conflicting language instructions in the same message.
Preserve the facts when switching language
Keep names for people, places and objects consistent. If a location has two possible translations, choose one. Add a brief scene recap when switching halfway through an established story.
For language practice, request a small correction after the answer rather than asking for a translation, grammar lesson and new plot in every turn. Independently check important translations when errors would matter.