Mixed-language interviews are common when HR works in German and the hiring manager works in English — or the reverse. Interviewers may change language to test how you handle real workplace communication.
Prepare core stories in both languages: a conflict you resolved, a project you finished under time pressure, and a decision you would make differently. Do not translate word for word; rebuild each story so idioms sound natural.
If you need a moment to switch, say so briefly. A calm pause is better than rushing into the wrong register. Ask which language they prefer for technical detail if the posting was unclear.
After the interview, send a short thank-you in the language that dominated the conversation, unless the recruiter wrote to you in the other language first.