To quote Proust: Life is too short and German too long.
Chinese is far, far easier than German. However if you are an adult you can make a choice, learn to read it or speak it. And yes, I’ve studied both German and Chinese.
The best language to study is the one you’re most likely to practice, all other things being equal. For most Yanks, that’s Spanish.
Other languages are good too, but Spanish is the easiest to pickup for basic speaking and writing. Learning to write Mandarin, by contrast, requires years of consistent, focused effort.
Kion pri Esperanto?
None, I’m just waiting until the iTranslator comes out. 
ETA: English is my second language, since I grew up in Rhode Island.
Would you like a cabinet or a regular coffee with your quahogs? (I’m from there, too, more or less. Lived more in Mass, but family from RI.)
These were the first that came to mind for me. The order, of course, would depend on where you live or are planning to go.
What do you want to do with the language? Talk to people at the local restaurant, go live in Germany, etc? It’s a really tough question to answer.
Chinese (Mandarin) is pretty awesome because you can go pretty much anywhere in the world and there is a Chinese restaurant and a Chinese business person and a Chinese trading company that does local business in the local language.
I remember being in the Loire Valley in france, in some non major town. My wife was sick and needed something to eat. I went out speaking practically zero french trying to figure out something my wife would eat. Found a Chinese restaurant, spoke Mandarin to the proprietor, was treated like a king, and got great takeout.
Chinese can be extremely rewarding depending on your interests and how you’re wired. But it ain’t easy.
China Guy - Chinese major in university, lived in Taiwan, Hong Kong and China for 20+ years, wife is Shanghaiese, 3 kids all born in China and first language was Chinese. Now live in suburbia US of A. Go figure
Esperanto, la alia monda lingvo?
I started learning Esperanto because it was easier than French. It turned out to be the log that broke the logjam of my monoliguality, and afterwards it was easier to resume French… and even go to French immersion.
Just sort of answered this in another thread. I would say Chinese, Japanese, Arabic or Moscovite-style Russian. But some missionaries I know insist that Portuguese is a pretty darn good one to have in a lot of the Third World areas.
Unless we are talking about Brazil (where it is useful), French will get you a LOT farther than Portuguese. It’s not even comparable. There are about 14 million Portuguese speakers in Africa, and it’s the official language in 5 countries (some of which are quite small). French is spoken by 180 million Africans in 25 countries.
I’ve worked for international companies for the bulk of my career. In almost any location, I’ve been able to connect with someone who speaks English, but if not, I can usually find someone who speaks French.
I presume we are talking about Sub Saharan Africa here, not the N African Arab states?
You would know better, but most of the Africans I have dealt with even from Francophone countries spoke excellent if accented English. I dealt with businessmen/women so maybe thats why.