My AI system kept randomly switching to French mid-answer and it took me way too long to figure out why

I built a RAG system that needs to answer in German or English depending on the query language. Sounds simple. It was not. The source documents are mostly in German but some contain French legal terminology, Latin phrases, and occasional English citations. What kept happening was the LLM would start answering in German, hit a French passage in the context, and just. switch to French mid-paragraph. Sometimes it would blend German and French in the same sentence. Once it answered entirely in Italian and I still have no idea why. I tried letting the LLM detect the query language itself.