@java.lang.Override
  public java.lang.String getSourceLanguageCode() {
    java.lang.Object ref = sourceLanguageCode_;
    if (ref instanceof java.lang.String) {
      return (java.lang.String) ref;
    } else {
      com.google.protobuf.ByteString bs = (com.google.protobuf.ByteString) ref;
      java.lang.String s = bs.toStringUtf8();
      sourceLanguageCode_ = s;
      return s;
    }
  }
<pre>
Optional. The ISO-639 language code of the input document if known, for
example, "en-US" or "sr-Latn". Supported language codes are listed in
Language Support. If the source language isn't specified, the API attempts
to identify the source language automatically and returns the source
language within the response. Source language must be specified if the
request contains a glossary or a custom model.
</pre>

<code>string source_language_code = 2 [(.google.api.field_behavior) = OPTIONAL];</code>

@return The sourceLanguageCode.
java-translate/proto-google-cloud-translate-v3/src/main/java/com/google/cloud/translate/v3/TranslateDocumentRequest.java