I had previously heard of Basic English, a proposed simplified English with an 850-word core vocabulary. (In case you're curious, a Basic English website has some examples.)
But I hadn't previously known that Basic English is just one example of a controlled natural language, a subset of a natural language "obtained by restricting the grammar and vocabulary in order to reduce or eliminate ambiguity and complexity."
And I also hadn't known about Special English, used by some Voice of America radio broadcasts to make them easier to understand by non-native English speakers. VOA has been broadcasting in Special English for almost forty years now.
Special English has a 1500-word core vocabulary. To quote the VOA site:
Special English writers use short, simple sentences that contain only one idea. They use active voice. They do not use idioms.
Special English broadcasters read at a slower pace, about two-thirds the speed of standard English. This helps people learning English hear each word clearly. It also helps people who are fluent English speakers understand complex subjects.
I think it's a neat idea, especially because these days VOA also posts corresponding written versions of the Special English broadcasts on their website, so listeners can compare the spoken and written versions, which a couple of listener letters on the abovelinked About page indicate can be helpful to people learning English.