Depends, mate, on where you live. Louisiana? Maybe French would be useful. California, Texas, Florida? Spanish is damn useful. I live in Miami and often have to deal with people that don't speak english. German is relatively common as well. And of course we are going to need people that speak Chinese and Japanese. Spanish is not too difficult (for me) while the pronounciations in French are nearly impossible. German is pretty complicated while the far east asians have a whole new system of writing for you to learn; that might be more than you want to take on. Do you already speak anything other than English, already? Anywhere intrigue you to travel too? Because the best way to learn a language is to get in country.
Spanish is, in my experience, the most grammatically simple, beautiful sounding and useful of the world languages.
That's a really good suggestion about Polish. Could be very useful in business. And it sounds absolutely gorgeous For travel, it obviously depends where you want to go
Hebrew is fairly simple, but not too useful. I would agree on Spanish, it is wonderful when you get past the grammar laws.