If the limit is 20 concurrent requests and not 1, we don't need to use synchronous requests. We only need to limit concurrency to 20.

Can you try forcing HTTP/1.1? Because the default is to open max 6 connections the remote hosts. If we hit the 20 limit, it means we're using HTTP/2. If that works, then the fix could be just to force thehttp_version in the constructor:
$this->client = $client->withOoptions(['http_version' => '1.1']);

Thanks for your suggestions, putting a limit of 6 or 20 concurrent requests won't really work in our case, since we pull translations locally, in our CI, and when deploying our app in review apps / staging / production.

Putting a limit of 1 would be ideal for us, this way we can be sure that we won't hit the Crowdin's concurrent requests limit across envs.

Maybe the provider should also wrap the client into a RetryableHttpClient

putting a limit of 6

Can you still give it a try? 3x6 = 18 so that might work.

Closing as we're likely not going with this approach. Please let us know if force http 1.1 is enough in practice.

Yeah I gonna try to force HTTP 1.1, or playing with max host connections option ASAP