English is the obvious choice for a inter-national language.
English is a hybrid language. Primarily between Latin-based Romance languages (blue on map) and Germanic languages (rust color). Romance language speakers will understand, in context, the English word liberty (Italian: libertà, French: liberté, etc). Germanic speakers will understand, in context, the English word milk (German: milch, Swedish: mjölk, etc)

In order to be hybrid, English has to be flexible with the order of nouns and verbs in sentences. English also dropped "gender" and other complicated language features. English adopts other languages' words when something new is labeled instead of creating its own word. Especially names of foreign objects. English speakers and the foreign object's natives can both refer to the object (for example, to trade ginseng). When precise scientific terms are needed English uses classic language roots (Latin, Greek). These roots are more likely to be intuitively understood by Indo-European language users.
English's flexibility and 800 basic words is enough for people to communicate fairly well. With context (e.g. pointing at an object) you can do international trade. This also allows pidgen versions of English to easily be created.
English is also capable of rapid language creation in subdomains. In-group slang is easily created. Domain specific technical terminology is easily created. Creative cultural works can imbue themselves with ensorcelled lexicography, multiplexed techno-babble, thine kyng's Ænglisc, kawaii desu, and straight up nigga lingo.
Everyone from the dumbest retard to the esoteric intelligentsia will find English very useful.
TL;DR: English be OP, simple as.