Well I found out my self. It is just about yielding at the return value from InvokeServerMethodAsync(), for example:
string returnText ="";
var googleTranslateOperation InvokeServerMethodAsyncString("C2NetServer.RpcMethods,C2NetServer",
"GoogleTranslate",
args => returnText = args.blabla, langId,englishText);
yield return googleTranslateOperation;
yield return Coroutine.Return(returnText);