There are global factors that affect everyone's choice of what foreign language to learn. Ultimately the choice depends most on individual circumstances, but the less gravitational pull someone has to any particular foreign language, the more relevant these factors are. Below is data to consider on the effort level each language requires, the population it has, and its international relevance.
The Foreign Service Institue of the U.S. Department of State has seventy years of experience training American diplomats for international assignments. They have put in place a language teaching system designed for their students to achieve a minimum proficiency level that is the same across all their languages. What is significantly different about each of the language programs, though, is the time it takes for the students of each language program to achieve that level of proficiency.
How much speaking opportunity a language will enable is of primary importance for most people when committing to a foreign language. What matters most to any language learner is how prevalent the language is close to their life, but here's a global look:
Depending on how active you are online, this may be a factor as well.
Typically, the larger the economy behind a language, the more professional opportunity there will be. As both a Mandarin and Spanish speaker, though, who has worked in (US-based) Chinese, Nepali, and Latin American companies, I would say English is by far the most consequential language for excelling in most of the hot industries. Innovation happens in relatively free countries and the good majority of them are Western. Speaking other languages has been more of a suplementary factor for me.
The most influential breakthroughs in thought are written down. Influential thought leaders write books with at least some differentiation from others. They may garnor followings in other mediums, but they refine their messages through writing. The amount a culture writes is a very strong indication for how much cultural development it accounts for. The more culture, the more their international influence and relevance. Here is current data on the top languages ranked by the amount of new thought they account for.