Russian

Adding Russian to your repertoire of languages will give you access to the treasures of Russian literature and open up a country of over 144 million, which after a severe downturn of the late 20th/early 21st century is reestablishing itself as a global economic and geopolitical player.

The language itself is used well beyond the borders of the motherland, with millions of native speakers of Russian residing in neighorboring countries, and further hundreds of thousands in the world's biggest English-speaking urban centers. You will never run out of conversation partners… or depressing literary masterpieces.