Add Language product slug (#3368)

It is a new service with a new product slug

eng/common/scripts/Test-SampleMetadata.ps1

@@ -221,6 +221,7 @@ begin {
         "azure-kubernetes-service",
         "azure-lab-services",
         "azure-language-understanding",
+        "azure-language-service",
         "azure-lighthouse",
         "azure-linux-vm",
         "azure-live-ondemand-streaming",