The reasons most programmers use English in their source code has nothing to do with file size (for that their are JS minimizes) or supported encodings. It has to do with that two things, English is the most used language in the industry so if you want to cooperate with programmers from other parts of the world English is a good idea and because it frankly looks ugly to mix languages in the same file so when the standard library is in English your source code will be too.

So since most source code is in English (and for JS is minimized) UTF-8 works perfectly there too.