Yes, a WCF client can work against a web service created with another technology or platform. It's fairly common. With Visual Studio, the client side proxy code is generated for you from the WSDL, and you can invoke a remote service as if it was a local code.
Not to pull a shameless plug, here's a sample of a WCF client calling the Amazon S3 web service, which is quite certainly not built with .NET. The sample demonstrates a typical workflow when creating a WCF client:
Add a Service Reference to the web service, by pointing Visual Studio at the WSDL URL. You don't have to code anything in this step.
Invoke the web service from your code. In the code below, the AmazonS3Client class was created by Visual Studio in step 1 above. You get full intellisense when typing client.ListAllMyBuckets to invke that service.
static void Main(string[] args) {
DateTime now = LocalNow();
// create the web service client object
AmazonS3Client client = new AmazonS3Client();
// invoke the web service
var result = client.ListAllMyBuckets(
accessKeyId,
now,
SignRequest(secretAccessKey, "ListAllMyBuckets", now));
// show the results returned from the web service
foreach (var bucket in result.Buckets) {
Console.WriteLine(bucket.Name);
}
}