This is perhaps one of the hardest questions to answer. So much depends on purpose, interests, prior experience and even mundane things like course availability and teacher-student rapport.
The correct answer is this one:
Many people will have a particular reason for learning a certain language. If you just have a desire to experience foreign language learning, it doesn’t really matter which one you choose. Which country or culture interests you?
BUT that’s not the answer that people want to hear. So the marketer in me often tries to understand in more subtle ways which language the potential client already has in mind. Often people already have an idea but are uncertain of whether their choice makes sense.
I often tell the story of how I figured out what people want from a language course. One day I fielded an enquiry from a young lady interested in learning French. We got talking about Language of desire why she wanted to learn French. The more we talked, the more realised that she wasn’t there simply because she wanted a language course. She told me how she wished to be fluent enough in order to watch without subtitles.
It dawned on me for the first time 3 or so years ago that the real reason many people wish to learn a language like French is because of the romance associated with it. This is sometimes subconscious, and often other more apparent and easy-to-explain reasons (like desire to travel, personal interest) come to mind.
So in future If you’re asked at a social function why you’re learning a certain language, just say that you’ve decided that you’ll do one thing this year that pushes you outside your comfort zone. It’s OK. You’re not the only one in that category. People may laugh at you, but secretly they admire your tenacity and resolve. Or just say that you feel it’s the romantic thing to do. Girls love that.