You basically cannot learn a foreign language after 16 as you can before.
Which one should you learn? Arabic or Farsi will help the most with world peace, which all of us are going to have to appreciate, at least a little. French and Italian are the prettiest. Spanish is the #2 language in America (where I suspect most of my readers reside). Chinese is the most widely spoken language on Earth. Latin and Greek both have special places in academia. Sanskrit, if you consider Panini, is undeniably special. Most of the Slavic languages are inter-intelligible, so if you learn Russian, you cover a lot of the world. Or, you might consider learning German, just to read Hegel and Goethe in the original.