Chinese languages

How difficult is it to learn Chinese for someone who is fluent in English? I also speak spanish too. Is it difficult considering you have to learn the spoken language and the Chinese characters? I know a few Chinese words but have never tried to learn more.

Which foreign languages are most sought after for jobs? Is English considered a foreign language in India? I've always wanted to visit India and think it would be fun to work there for a year or two.

Is Mandarin a dialect like a southern dialect in the U.S.? Or is it different characters and words completely? How many different dialects exist in the country of China and which one is more common?

I wish I would have learned more foreign languages when I was young. I learned a little Spanish and German but can't speak either well. One of my friends took Chinese in college and he said it was really difficult.

What language is the most spoken in the world? I always thought it was Spanish with English coming in second. I never realized English was spoken in so many countries.