External Servers have nothing to do with external language servers.
We're supposed to call them "External Servers", but they were called "External <Language> Servers" during development (where <Language> is shorthand for <Java | .NET | Python>). The habit of using the old terminology is hard to break.
The $system.external interface is documented in Using InterSystems External Servers (not "external language servers", which are extensions to editors like VSCode).
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