In our opinion, the five most useful languages for a world traveller to learn are English, Spanish, French, Chinese and Arabic – ranked in decreasing order of usefulness.
These languages are spoken as either first or second languages in multiple countries, by hundreds of millions of people across diverse regions of the world.
A distinctive feature of the first group (English, Spanish and French) as opposed to the second group (Chinese and Arabic) is that you can speak English, Spanish, or French in almost any popular dialect or accent and still be intelligible anywhere the language is spoken. This is because the differences among the dialects are only very slight.
Conversely, Chinese and Arabic have many forms and dialects that are very different, so much so that some of them are mutually unintelligible. That is why we rank them 4th and 5th respectively. Where these two languages are concerned, it is therefore advisable to learn the most widely spoken forms. For example, Mandarin and Cantonese are the two most useful forms (or dialects) of Chinese language to learn for a traveller.