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If you want to learn any language and aren't interested in how widely it is spoken I'd recommend one of the Scandinavian languages. I speak German and English fluently and I just started learning Swedish 2 weeks ago. I can already form basic sentences and have short, simple conversations. Of course it depends on how well you speak German, but if you're almost fluent learning a Swedish is dead easy. Most of the grammar and words can either be found in English or German. So if it's just about learning any language really quickly then try out Swedish or another Scandinavian language (except Icelandic which is really hard).

Any language based on latin: best is french, when you can speak this spanish and italian are easy to learn as all languages are based on latin and the "core-word" is always latin, so even if you can not speak it you can read it and understand it

Either French or Italian...right next to Germany

Arabic... Reading and understanding arabic is easier than many would think, despite it being difficult to compose arabic sentences if you are not much familiar with the language.