Mandarin, English, Spanish and Hindi.
All these languages have a high amount of native speakers so you might want to try learning one of them through learning apps like Duolingo or by watching movies or other shows in the languages.
Learning english will be better since it's a fast growing language and is certified in almost all the countries in the world. Mandarin (China, Taiwan, Hong-Kong, Macau), Hindi (India), and Spanish (Spain, Latin America) on the other hand have a large amount of speakers since they are spoken in populated countries but not everyone might know these languages outside these regions.
