I don't think you really have a choice TBH. Trying to do something like that sounds like a world of pain, and a bunch of unidiomatic code. If you can't actually support 4 to 10 languages, maybe you should cut back on which ones you support?
One interesting thing you could try if you really don't want to cut back is to try having using an LLM to take your officially supported code and transliterate it to other languages. I haven't tried it at this scale yet, but LLMs are generally pretty good at tasks like that. I suspect that would work better than whatever templating approach you've used before.
If neither of those approaches works, everything speaks C FFI, and Rust is a modern language that would work well for presenting a C FFI that the other languages can use. You're probably not hot on the idea of rewriting your Go tests into another language, but I think that's your only real option then.