It's a difficult question, but I would say expertise in a second language should be first place when I begin learning. Learning a second language is the first experience for learners, so they discover many challenges and successes throughout learning. I think this is why people want to learn other languages.
I thought the case if I learned various languages at once. It seemed like fun at first, but I would be exhausted soon. It might be possible if I were a linguistics student or had sufficient time and passion for learning.
I'm learning English as a second language. If I can get fluent in English, I want to try to learn another language.