Polish and slavic languages I think are over estimated. They have pretty strict grammar rules, so once you get over that first hurdle its all vocabulary from there on bar a few complex grammar rules and of course some exceptions that only fluent/near native speakers will remember.
When I was choosing a language, I decided to choose one that was common, but also surprising for the native speakers to here a foreigner speak. Not exactly froma business point of view, but go to a russian speaking country and speak russian and the girls love it. I cant imagine french, german etc girls being so easily impressed, or impressed so much that they will umm...
As for learning, I would say find a class, the pressure from the teacher and not wanting to look like an idiot infront of the teacher and classmates is a great incentive to learn. If thats not an option, look up the university league tables for that language then check what the recommended course text book is, most of them will come with audio supplementry material. Just make sure you get something which tests/quizzes you so its not all reading and memorising.
