Hi - I've built something for this that avoids the Google API key problem entirely, and I've tested it on a live server.
It supports two options:
MyMemory - no account, no key, no card. Free tier is 5,000 characters a day, or 50,000 with an email in the config. Fine for a quieter server.
LibreTranslate self-hosted - this one does the translating on your own machine rather than calling out to anyone, so there's no limit and no third party involved. Chat never leaves your server.
Before I say anything more definite: does your Rust server run somewhere you can install Docker? That decides whether the unlimited option is open to you, or whether you'd be on the free tier.
Happy to send you a copy to try either way.
