But what is an API? Is it just a set of functions? Even then, it is already a language, with its own vocabulary. Or, is it a set of classes, interfaces, etc.? Then, even more so. And so on. Limitations and particulars of the syntax of a host language do not matter as much as you may think. Also, if you look close enough, you will realize that most of the languages already are, in fact, combinations of various built-in sublanguages, each having its own "subsyntax". What matters is the ease with which the host allows to create a new sublanguage that suits a particular purpose.