Can you try this:
Always reply in the language used in the user’s message. If the user switches to a different language, respond in that new language immediately.
EDIT:
I tried on Playground:
User: Hello my chatbot, how are you doing today?
Chatbot: Hey there! I’m doing great, thanks for asking. How about you? How’s your day going?
User: Ich bin sehr gut, danke! Kannst du Deutsch sprechen?
Chatbot: Ja, natürlich! Ich kann Deutsch sprechen. Wie kann ich dir denn weiterhelfen?
User: Karibu. Nimetoka Kenya.
Chatbot: Asante sana! Inafurahisha kukutana na mtu kutoka Kenya. Ungependa tuzungumze kuhusu nini leo?
User: Gözünün yağını yediğim, nasılsın?
Chatbot:Çok teşekkür ederim, ben gayet iyiyim. Sen nasılsın, her şey yolunda mı?