There are several reasons.... and it is totally false.
The first, reason: the most important one is that since the end of WWI, english have been the most used international language in North America, West Europe and Japan; as most (cf reason #2) language building have been done since the 40s, english was favored, and that most of the practical development have been done in the US.

The second reason is that "informatics language" is just a way to means "algorithmic language that my advanced adding machine can understand", which means the first informatics languages predates the first computer from more than a century: the work of Babbage and Lovelace for example, were in English; the industrial sewing machines, which appeared in England in the 18th century were a sort of computers, which accepted instructions in the form of perfored cards and had a carpet-screen (i.e. the result of the algorithm/data on the perfored cards was posted, not on a screen, but on/in/as a carpet).

The third reason is that english provides "easy" words : I learned a french-based Basic version as my first programming language and the "Jusqu'a" clause, I think, would be an horror for non-french speaker or even for them...

Now why the assertion is false: not all languages are in english. Those used the most, in their most used version, are, that's perfectly true. Although there are many, many languages not based upon english, most of them are not used at all, and never were. But some were and still are: some languages were created based over russian in the Soviet Union (and still are used because rebuilding a whole system is just hard) for example; the only recent non-english based language I know is Wlangage (which exists in and can even mix french, english and chineese... as if the mixing part was really necessary but still...), and it can be quite weird (singular and plural are important for example. or verbs).

And there are even languages which are not based upon any language. The most important is machine code, which is used thanks to an english based assembly language but is basically the core of any and all languages (remember: a language is just the way one give orders to a very dumb and very efficient adding machine, which understand only 0s and 1s (and in the case of Bender, at least a 2).