I think it depends very much on what you're doing. For a lot of uses of extension languages, you might not need third-party libraries or even a large standard library. But if you need, say, cryptographic algorithms, you probably don't want to implement them yourself, you'd rather use battle-tested (and perhaps externally validated) implementations maintained by security experts.
Plus, the standards and protocols that your code might need to deal with are going to change over time. Yesterday it was Unicode, today it's gRPC and Kafka, tomorrow it will be something else.
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