At a couple of bookshops with dedicated areas for language learning, some languages get entire aisles, while other languages get just one or two books.
The number of speakers can't be the sole determining factor of a language. For example, Mandarin Chinese has far more speakers than English, but many countries put more of an emphasis on learning the latter, even China's neighbour Japan.
I don't have any professional reasons to learn a language, and don't currently have any personal reasons to do so either.
When deciding whether or not to learn a language, or which language to learn next, I'd like to have some objective data along with my own personal preferences.
What metrics or other means can I use to evaluate the importance of a language?