Which language is worth to master except English?

i'd say chinese (mandarin). its the largest country in the world and growing more influential every day

Chinese...as China will become the greatest influence in economy market soon..

As a GERMAN teacher, obviously I have a bias, but I would still say the language has to "speak" to you; if you are not "drawn in" by it, you won't learn it effectlively. Years ago, I discovered that German was the language that fascinated me and I've never regretted learning it. FInd your own passion.

French then you know what you're eating when you have some posh grub Unless you play the piano when Italian would be useful .

I think it would be Chinese or Japanese. There are alot more Asians out there and those two are currently showing the most promise, Japan showing the most promise in technology, research, and energy efficiency and China being by shear number and potential as a rising super power... Spanish is a close 3rd but that just applies to people on a more local basis in the U.S if your closer down south. Other European languages such as French and Italian are only a good idea if you live in a neighboring country that speaks it... I personally prefer latin and sanskrit as someone already mentioned above me as they are root languages most modern languages are based on with major influences...

Japanese

try latin or sanskirt they were the greatest language and the most influential but if you want something easy go for french it's worth and if you are looking for something difficult try japanese.

i will recommend you learn mandarin

German is really easy to pick up. But i find it more satisfying to learn Spanish for some reason.. The language of love i guess. =D Good luck.