If you make regular visits to, or have regular contact with people from a particular country or region, learning the language(s) spoken there would be a good idea.
If you have an interest in some aspect of a particular culture, such as literature, music, art or food, learning the language of that culture will help you explore your interest in great depth.
If you want to learn a language that is ‘useful’ generally, then the ones to consider are French, Spanish, Russian, Arabic, Chinese (Mandarin), German, Japanese, Portuguese and Hindi/Urdu. This list is based on the number of speakers, the number and population of countries where the languages are spoken, the number of major fields using the languages internationally, the economic power of countries using the languages, and their socio-literary prestige.
Some languages might be more in demand than others in your region. Looking at job advertisements, particular those on recruitment agencies specialising in bilingual and multilingual jobs, is a good way to find out what employers are looking for.
Another factor to consider is whether courses and materials are available for the language you want to learn. This shouldn’t be a problem for popular languages like French, German and Spanish, but it might be more difficult to find courses in less commonly-studied languages such as Hindi or Malay.
Some languages are more difficult to learn than others. If your native language is English, the least difficult languages to learn are French, Spanish and Italian. However, if you relish a challenge, languages like Chinese and Japanese are worth considering.