✔ 最佳答案
First of all, I want to compliment you on your English fluency. You write better than most people for whom English is their mother tongue.
Your question asked specifically about which "is the most useful language in the world". You didn't ask what what languages Americans should learn in addition to English. Nor did you ask what language is the second most useful in the world. The direct answer to your stated question is, undoubtedly, English.
All international pilots must speak English fluently. Nearly all international scientific symposiums are conducted in English. Most international business transactions are conducted in English. It is the most widely spoken language in the world being solidly established on all seven continents. Swedish, for instance, is not normally taught in African schools but English is usually a required subject.
Now, if you are asking about another language to learn in addition to Chinese and English, the answer depends upon which source you use. The Summer Institute for Linguistics (SIL) Ethnologue Survey (1999) lists Spanish as having the second highest number of native speakers (after Chinese) at about 332,000,000. In terms of gross numbers and not taking into account population concentrations, so long as you speak Chinese, English, and Spanish you are able to communicate with approximately 1.6 billion people, or about 23% of the world's population. That's not bad! :D
Personally, I speak English, Spanish, and am functional in German. French completely eludes me. I guess there just isn't enough French spoken here in Texas for me to get a grasp on it. That being said, I have travelled extensively throughout the Western hemisphere without difficulty.
I admire your desire to better yourself. I wish that more Americans would show the drive and desire that you obviously have.