diff --git a/google/cloud/texttospeech/v1beta1/cloud_tts.proto b/google/cloud/texttospeech/v1beta1/cloud_tts.proto
index 4655acfa..bf4726b8 100644
--- a/google/cloud/texttospeech/v1beta1/cloud_tts.proto
+++ b/google/cloud/texttospeech/v1beta1/cloud_tts.proto
@@ -30,19 +30,32 @@ service TextToSpeech {
   // Returns a list of [Voice][google.cloud.texttospeech.v1beta1.Voice]
   // supported for synthesis.
   rpc ListVoices(ListVoicesRequest) returns (ListVoicesResponse) {
-    option (google.api.http) = { get: "/v1beta1/voices" };
+    option (google.api.http) = {
+      get: "/v1beta1/voices"
+    };
   }
 
   // Synthesizes speech synchronously: receive results after all text input
   // has been processed.
   rpc SynthesizeSpeech(SynthesizeSpeechRequest) returns (SynthesizeSpeechResponse) {
-    option (google.api.http) = { post: "/v1beta1/text:synthesize" body: "*" };
+    option (google.api.http) = {
+      post: "/v1beta1/text:synthesize"
+      body: "*"
+    };
   }
 }
 
 // The top-level message sent by the client for the `ListVoices` method.
 message ListVoicesRequest {
-
+  // Optional (but recommended)
+  // [BCP-47](https://www.rfc-editor.org/rfc/bcp/bcp47.txt) language tag. If
+  // specified, the ListVoices call will only return voices that can be used to
+  // synthesize this language_code. E.g. when specifying "en-NZ", you will get
+  // supported "en-*" voices; when specifying "no", you will get supported
+  // "no-*" (Norwegian) and "nb-*" (Norwegian Bokmal) voices; specifying "zh"
+  // will also get supported "cmn-*" voices; specifying "zh-hk" will also get
+  // supported "yue-*" voices.
+  string language_code = 1;
 }
 
 // The message returned to the client by the `ListVoices` method.
diff --git a/google/cloud/texttospeech/v1beta1/texttospeech_gapic.yaml b/google/cloud/texttospeech/v1beta1/texttospeech_gapic.yaml
index fb9485f9..44667673 100644
--- a/google/cloud/texttospeech/v1beta1/texttospeech_gapic.yaml
+++ b/google/cloud/texttospeech/v1beta1/texttospeech_gapic.yaml
@@ -115,6 +115,10 @@ interfaces:
   #   the call is retrying, refer to retry_params_name instead.
   methods:
   - name: ListVoices
+    flattening:
+      groups:
+      - parameters:
+        - language_code
     request_object_method: true
     retry_codes_name: idempotent
     retry_params_name: default