I don't know if this question is too subjective or vague, but I don't really know what I could answer if I had to. I am a junior developer (I had a couple of internships which were great and got offered job opportunities), but when I was asked this by another company's HR, I didn't know what to say, I just stuttered something like "Well, writing code in a whole new language seems like a great challenge to me, but I don't know if it would be the greatest".
The interviewer told me back that as a developer, I had to experiment a lot, and try new languages, frameworks and such. I'm perfectly aware of this, I'm curious and try plenty of those, but I didn't even know what to answer because I'm not shy at all, I work great in a team and I'm quite organized. Thus answering "There is no great challenge for me" really didn't seem like an acceptable answer.
What is the interviewer looking for by asking this question, and how can I craft a good answer for this question in the future?