As far as I know there’s no dedicated test API key for Google Cloud Translation API. But there’s some strategies to mitigate costs during your development like applying API key restrictions such as IP addresses and HTTP referrers. For example when creating your API key, restrict access to specific IP addresses, This limits usage to your local development machines or your test server's IP and prevents accidental or unauthorized access from other sources.

But if you just really want to do a lot of API Key testing, then maybe the 90-day $300 free trial for Google Cloud is another option for you.