What European language shall I learn?

spanish, because it is one of the world's most spoken languages (considering the amount of countries with spanish as its native language) besides spanish also provides an opportunity  to understand related languages such as portuguese, italian and even a bit of french.

It depends on what you intend to do : - if you want to learn a language for fun, try French, Spanish or Italian then you'll enter a very interesting poetry and literature - if you want to understand the secret of German organization and industry efficiency learn German - if you want to travel to Eastern Europe German will be a real asset - if you want to travel in Northern Europe English will be enough, but Swedish will help read and speak a little with people at least in Sweden and Finland - if you intend to get the melancholy of Portuguese "Saudade" and then travel to Brazil, learn Portuguese - and so on, and on again

I would suggest Spanish, French, and German in that order. Spanish would open up Spain and the whole of Latin America. French - France and a whole list of French-speaking countries - scattered across Africa and the rest if the world German - well, you can always do great things with the Germans. After all, they won the World Cup.

has already covered the best. I will add a few pointers for the prioritizing Spanish over everything else. If you look into the number of people in the world who speak Spanish, the % is slightly above English. Infact, Spanish has the maximum number of native speakers in pure numbers after Mandarin. The ability to speak Spanish will aid your communication in more than 15 countries of the world. That is really huge :)  Spanish and English together would help you in communicate with ~11% of the entire world population! (with the population base of 7BN ->  this number is really huge) Secondly, Spanish is very much related to Italian and Portuegese. A lot of words and sentence construction rules similar in these languages makes people who speak Spanish easily understand quite a bit of Italian and Portugese. And additionally Portuguese is 6th in the world by the total number of native speakers.

French is one of the most popular languages. French is the first or second language in more than 40 countries and is spoken by 125 million people on every continent. It is spoken in France, Canada (Quebec), Most of the African countries and some islands. It is a beautiful language and the literature, and the artists of course have been spectacular. First and foremost, learning French is the pleasure of learning a beautiful, rich, melodious language, often called the language of love. French is also an analytical language that structures thought and develops critical thinking, which is a valuable skill for discussions and negotiations.I have learnt French. I liked the language so much.Other than French, I would suggest Spanish and German.

English, which you already seem to speak. Apart from that, in percentage of European population and keeping knowledge of English in mind. I'd say French. In my experience, German-speaking nations seem to have a better grasp of English than their French-speaking counterparts (whose english is by no means bad). It's also a good basic introduction to latin-based languages like Italian and Spanish. After that it would be German. But normally, English would be sufficient when it comes to your career. Europeans have had to work together for decades and the language usually chosen is recent years is English.