Online Innovation & Test'n'Target - Dell EMEA Online
Round Rock, Texas
So Mandarin would be a logical choice, due to the largest amount of speakers. But for Western or European locations, English is a very good base language to have - and a lot of international companies use English as their main language for communication...
But Latin-based languages are easier to transfer learning techniques and your ear can pick up 'other' latin languages. You should consider what you want to 'DO' with your language choice though. There's great opportunity in learning languages that are somewhat rare, or not commonly spoken - as you can be really specific in a skillset then (using your language skills). So consider the country/language and then tasks or jobs or opportunities that can stem from there!