Cloud Natural Language API: what are accessible languages?

Question

I'm trying to run Natural Language API with Google Sheets using this documentation. Natural Language API demo has a list of possible languages, which lists German too. But if I set the language in Google Script as de-de I get an error 400:

Exception: Request failed for https://language.googleapis.com returned code 400. 
Truncated server response: 
{ "error": { "code": 400, "message": "The language de is not supported for entity_sentiment analysis.", 
"status": "INVALID_ARGUMENT" ... (use muteHttpExceptions option to examine full response).

Do I do something wrong? Or is everything correct and this kind of analysis doesn't work with German?

Edit: This is the script I use. The language setting in the line 118 is currently empty: language: '',.

A funny effect I can't explain by myself: even without an explicit language setting the script/the API seems to recognize the language of the text input - whily trying to analyze a german text I get an error message The language de is not supported for entity_sentiment analysis. with or without language setting.

// To learn how to use this script, refer to the documentation:
// https://developers.google.com/apps-script/samples/automations/feedback-sentiment-analysis

/*
Copyright 2022 Google LLC

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

// Sets API key for accessing Cloud Natural Language API.
const myApiKey = 'MY_API_KEY'; // Replace with your API key.

// Matches column names in Review Data sheet to variables.
let COLUMN_NAME = {
  COMMENTS: 'comments',
  ENTITY: 'entity_sentiment',
  ID: 'id'
};

/**
 * Creates a Demo menu in Google Spreadsheets.
 */
function onOpen() {
  SpreadsheetApp.getUi()
    .createMenu('Sentiment Tools')
    .addItem('Mark entities and sentiment', 'markEntitySentiment')
    .addToUi();
};

/**
* Analyzes entities and sentiment for each comment in  
* Review Data sheet and copies results into the 
* Entity Sentiment Data sheet.
*/
function markEntitySentiment() {
  // Sets variables for "Review Data" sheet
  let ss = SpreadsheetApp.getActiveSpreadsheet();
  let dataSheet = ss.getSheetByName('Review Data');
  let rows = dataSheet.getDataRange();
  let numRows = rows.getNumRows();
  let values = rows.getValues();
  let headerRow = values[0];
  
  // Checks to see if "Entity Sentiment Data" sheet is present, and
  // if not, creates a new sheet and sets the header row.
  let entitySheet = ss.getSheetByName('Entity Sentiment Data');
  if (entitySheet == null) {
   ss.insertSheet('Entity Sentiment Data');
   let entitySheet = ss.getSheetByName('Entity Sentiment Data');
   let esHeaderRange = entitySheet.getRange(1,1,1,6);
   let esHeader = [['Review ID','Entity','Salience','Sentiment Score',
                    'Sentiment Magnitude','Number of mentions']];
   esHeaderRange.setValues(esHeader);
  };
  
  // Finds the column index for comments, language_detected, 
  // and comments_english columns.
  let textColumnIdx = headerRow.indexOf(COLUMN_NAME.COMMENTS);
  let entityColumnIdx = headerRow.indexOf(COLUMN_NAME.ENTITY);
  let idColumnIdx = headerRow.indexOf(COLUMN_NAME.ID);
  if (entityColumnIdx == -1) {
    Browser.msgBox("Error: Could not find the column named " + COLUMN_NAME.ENTITY + 
                   ". Please create an empty column with header \"entity_sentiment\" on the Review Data tab.");
    return; // bail
  };
  
  ss.toast("Analyzing entities and sentiment...");
  for (let i = 0; i < numRows; ++i) {
    let value = values[i];
    let commentEnCellVal = value[textColumnIdx];
    let entityCellVal = value[entityColumnIdx];
    let reviewId = value[idColumnIdx];
    
    // Calls retrieveEntitySentiment function for each row that has a comment 
    // and also an empty entity_sentiment cell value.
    if(commentEnCellVal && !entityCellVal) {
        let nlData = retrieveEntitySentiment(commentEnCellVal);
        // Pastes each entity and sentiment score into Entity Sentiment Data sheet.
        let newValues = []
        for (let entity in nlData.entities) {
          entity = nlData.entities [entity];
          let row = [reviewId, entity.name, entity.salience, entity.sentiment.score, 
                     entity.sentiment.magnitude, entity.mentions.length
                    ];
          newValues.push(row);
        }
      if(newValues.length) {
        entitySheet.getRange(entitySheet.getLastRow() + 1, 1, newValues.length, newValues[0].length).setValues(newValues);
      }
        // Pastes "complete" into entity_sentiment column to denote completion of NL API call.
        dataSheet.getRange(i+1, entityColumnIdx+1).setValue("complete");
     }
   }
};

/**
 * Calls the Cloud Natural Language API with a string of text to analyze
 * entities and sentiment present in the string.
 * @param {String} the string for entity sentiment analysis
 * @return {Object} the entities and related sentiment present in the string
 */
function retrieveEntitySentiment (line) {
  let apiKey = myApiKey;
  let apiEndpoint = 'https://language.googleapis.com/v1/documents:analyzeEntitySentiment?key=' + apiKey;
  // Creates a JSON request, with text string, language, type and encoding
  let nlData = {
    document: {
      language: '',
      type: 'PLAIN_TEXT',
      content: line
    },
    encodingType: 'UTF8'
  };
  // Packages all of the options and the data together for the API call.
  let nlOptions = {
    method : 'post',
    contentType: 'application/json',  
    payload : JSON.stringify(nlData)
  };
  // Makes the API call.
  
  let response = UrlFetchApp.fetch(apiEndpoint,nlOptions);
  return JSON.parse(response);
};