From personal experience the most useful language when traveling in Asia is Japanese. When I was visiting South Korea, I was denied entry at a restaurant by a nervous waitress who told me that she wasn’t able to serve me for fear of a lack of communication, and that if my only language was English she would not be able to serve me.
It was because I was able to speak, read, and write in Japanese that she allowed me to be seated and served. I was able to read, understand, and order off the menu smoothly and without any problems. In fact the service they provided to me was excellent, as well as the food. I dined alone, but enjoyed myself immensely and was treated royally.
Nine hundred million people from all over the world speak, or have some command of the English language. English is the language of business and commerce in Asia, it is also the language of Hollywood with its movies and music. English is versatile, dynamic, and cool. In Asia, Asian businessmen and merchants want to be seen and heard using English. It’s a social status issue. If you are able to speak English fluently, you are seen as highly educated, refined, and modern thinking.
However, when it comes to traveling and dealing with local merchants the Japanese language tends to be one of the more popular languages to converse in. This could be because many Asian countries were former colonies of the Empire of Japan and Japanese was forced upon the locals. When Japan reemerged as an economic world power the popularity of Japanese took off and became necessary to learn as many Asians flocked to Japan to either live or seek a new life. Speaking English is more about appearing cool and fashionable whereas learning Japanese is a matter of survival.
In Japan there’s Wasaeigo. In Singapore we have Singlish, and so on. These are heavily modified forms of the English language which often times irritate native English speakers. Wasaeigo ( Japanese-English) (Singlish-Singaporean-English) often use broken and inflected and sometimes confuses North American English speakers. The interesting thing here is that in countries like Singapore, China, and Korea the Japanese language is rarely ever butchered, reinvented, or misspoken. It is always clear and respectful.
If you are planning a trip to Asia and are not sure which language is best to learn, I recommend learning Japanese. If learning languages is not something you are interested in doing, English should suffice most of the time, especially in major cities with lots of touristy areas.