At the end of the day, your goal is to engage with your interviewee meaningfully, so you can learn more about a particular subject. Using your second language helps establish familiarity with them, so they can share their perspective comfortably and honestly. In the preparation process, it is easy to get stuck in nailing every syllable, in every phrase and in every question — but that’s not the end goal. Your interviewee don’t expect you to be fully fluent in your second language, practice as much as you can, but it is just as important to set expectations with them upfront — that you aren’t the most proficient at it. This gives you room to pause, think and even make mistakes. By setting expectations upfront, you are also inviting your interviewee to partake in your thought process, which may yield insights that you may not normally gain in a normal conversation in your primary language.
In my interviews earlier this week, I stumbled to communicate the name of a key feature. Flustered and nervous, my interviewee then asked me what it is for. By explaining the outcome and purpose, we were able to work back towards the particular name, and even came up with a better label for it.