The GPL applies to "derivative works", which is a legal term without a precise technical definition. If you decide to write an interpreter for an existing lang

The GPL applies to "derivative works", which is a legal term without a precise technical definition. If you decide to write an interpreter for an existing language, and run existing programs on it, then those existing programs "probably" do not have to be under the GPL, for definitions of "probably" that Legal will not be happy about. If you write an interpreter for a made-up language and then write programs in that language as part of a thinly-veiled scheme to do an end-run around the GPL, then that "probably" violates the GPL. There is currently no bright-line rule separating those cases; this is something IP lawyers argue about themselves and reach wildly different conclusions.