One aspect that many language learners consider is "how often can I use the language?". This is probably because those who spend years studying a language want it to be useful (although various aspects go into the usefulness of a language). Such individuals may want to look into Chinese ( 汉语/漢語 Hànyǔ or 中文 Zhōngwén). According to "Nations Online" there are 1,146,755,000 first language and 1,343,755,000 second language Chinese speakers. The second most popular language is debated. Some sites put English, others put Hindi, others say Spanish. However, all of the sources I went to put the macro language Chinese as the most widely spoken all over the world. Therefore, if you are debating about which language to learn (or learn next), some form of Chinese will allow you to converse with millions of people.
****Disclaimer: I do not believe in learning a language just because it is widely spoken. All of the languages I want to learn are spoken by my family. However, some individuals base their studies off of which language is most applicable.*****