Ask for the reply language
Tell the character the language and style you want. If a conversation has been in another language, it can take a few exchanges to settle into the new preference. Repeat the request alongside the immediate question if a reply switches back.
Please reply in Spanish, using short everyday sentences. Keep the character's name unchanged and ask one question at a time.
Keep the scene consistent
Use the same names for places, people and important objects. If a proper noun has two translations, choose one and keep using it. Supply a short recap when changing language halfway through a story.
For practice, ask for a small correction after the reply rather than mixing a grammar lesson, translation and a new story direction into every message. Check important translations independently when accuracy matters.
Interface and voice are separate
Use the Language control for the interface options offered by the current version. A character answering a language request does not establish that every menu, media tool or voice supports that language.
Do not reset or delete a conversation just to test a language change if you want to keep its context. Use a clear instruction first, and review reply-style examples if the scene becomes difficult to follow.