Any new programming language needs to implement libraries for things like JSON, Yaml, HTTP, MQTT, TLS, URL-parsing, etc.
Are there some good test-suites that explicitly test correct behavior for the most important RFCs? Maybe even an online service or docker container?
Is there an implementation that made these tests in such a way that they can be easily adapted for other programming languages?
You can find an extracted, installable version here: https://github.com/rogpeppe/go-internal/tree/master/cmd/test...
For test cases, I would look to libraries in any language that has them in an easy to copy format