Google Apps Script files: https://script.google.com/macros/s/AKfycbxYTTMS5XJY48xiVKkB1tWMXRowcjkUTuwkA93-4cU/dev

_script.html

<script src='//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js'></script>
<script>
function download(blob, filename) {
  var objectUrl = (window.URL || window.webkitURL).createObjectURL(blob),
           elem = document.createElement('a'),
          event = document.createEvent('MouseEvent');
  elem.download = filename;
  elem.href     = objectUrl;
  event.initEvent('click', true, true, window, 1, 0, 0, 0, 0, false, false, false, false, 0, null);
  elem.dispatchEvent(event);
}

function toBlob(base64, contentType) {
  var bin    = atob(base64.replace(/^.*,/, '')),
      buffer = new Uint8Array(bin.length);
  for (var i = 0; i < bin.length; i++) {
    buffer[i] = bin.charCodeAt(i);
  };
  try {
    return new Blob([buffer.buffer], { type: contentType });
  } catch (e){
    return false;
  }
}

$(function() {
  $('form').on('submit', function (e) {
    e.preventDefault();
    var params = {
      originalText: $('#originalText').val(),
      translateInto: $('#translateInto').val(),
      success: function (res) {
        console.log(res);
        download(toBlob(res.byte, res.contentType), res.name);
      },
      failure: function (res) {
        console.warn(res);
      }
    };
    google.script.run
      .withSuccessHandler(params.success)
      .withFailureHandler(params.failure)
      .onFormSubmit(params.originalText, params.translateInto);
  });
});
</script>

_stylesheet.html

<link rel="stylesheet" src="//normalize-css.googlecode.com/svn/trunk/normalize.css" />
<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" />
<style>
html, body {
  font-familiy: "Helvetica Neue",Helvetica,Arial;
  -webkit-font-smoothing: antialiased;
}
textarea {
  resize: vertical;
}
.navbar {
  border-radius: 0 !important;
}
</style>

application.gs

function doGet() {
  return HtmlService.createTemplateFromFile('layout').evaluate().setSandboxMode(HtmlService.SandboxMode.IFRAME).setTitle('Translate sentences in English');
}

function include(filename) {
  return HtmlService.createHtmlOutputFromFile(filename).getContent();
}

function onFormSubmit(originalText, translateInto) {
  try {
    if (!originalText && !translateInto) throw new Error('Missing required arguments');
    var data = translateSentencesFromOriginalText(originalText, translateInto),
        blob = generateCsv(data);
    return {
      byte: Utilities.base64Encode(blob.getBytes()),
      name: blob.getName(),
      contentType: blob.getContentType()
    };
  } catch(ex) {
    return ex.toString();
  }
}

function translateSentencesFromOriginalText(originalText, translateInto) {
  var result    = [['en', translateInto]],
      sentences = originalText.replace(/(\.+|\:|\!|\?)(\"*|\'*|\)*|}*|]*)(\s|\n|\r|\r\n)/gm, '$1$2|').split('|');
  for(var i = 0; i < sentences.length; i++) {
    var translatedSentence = LanguageApp.translate(sentences[i], 'en', translateInto);
    result.push([sentences[i], translatedSentence]);
  }
  return result;
}

function generateCsv(data) {
  var filename  = 'translate_from_english_'+new Date().getTime()+'.csv',
      csvString = Object.keys(data).map(function(i) {
        return Object.keys(data[i]).map(function(j) {
          return '"' + data[i][j] + '"';
        }).join(',');
      }).join('\r\n'),
      blob = Utilities.newBlob('', 'text/csv', filename).setDataFromString(csvString, 'utf-8');
  return blob;
}

function doPost(e) {
  try {
    if (!e) throw new Error('Missing required parameters');
    var data = translateSentencesFromOriginalText(e.parameter.originalText, e.parameter.translateInto),
        blob = generateCsv(data);
    return makeResponse(blob.getDataAsString(), ContentService.MimeType.CSV);
  } catch(ex) {
    return makeResponse(JSON.stringify({ message: ex.toString() }), ContentService.MimeType.JSON);
  }
}

function makeResponse(content, mimeType) {
  return ContentService.createTextOutput(content).setMimeType(mimeType);
}

layout.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <?!= include('_stylesheet'); ?>
  </head>
  <body>
    <nav class="navbar navbar-inverse">
      <div class="container-fluid">
        <div class="navbar-header">
          <a class="navbar-brand" href="#">Translate sentences in English</a>
        </div>
      </div>
    </nav>
    <div class="container-fluid">
      <form>
        <div class="form-group">
          <label for="originalText">Original Text</label>
          <textarea name="originalText" id="originalText" class="form-control" rows="10" placeholder="Please enter the text (in English) that you want to translate."></textarea>
        </div>
        <div class="form-group">
          <label for="translateInto">Translate into</label>
          <select class="form-control" id="translateInto">
            <option value="ar">Arabic</option>
            <option value="bg">Bulgarian</option>
            <option value="bn">Bengali</option>
            <option value="ca">Catalan</option>
            <option value="cs">Czech</option>
            <option value="da">Danish</option>
            <option value="de">German</option>
            <option value="el">Greek</option>
            <option value="es">Spanish</option>
            <option value="eu">Basque</option>
            <option value="fa">Farsi</option>
            <option value="fi">Finnish</option>
            <option value="fil">Filipino</option>
            <option value="fr">French</option>
            <option value="gl">Galician</option>
            <option value="gu">Gujarati</option>
            <option value="hi">Hindi</option>
            <option value="hr">Croatian</option>
            <option value="hu">Hungarian</option>
            <option value="id">Indonesian</option>
            <option value="it">Italian</option>
            <option value="iw">Hebrew</option>
            <option value="ja" selected>Japanese</option>
            <option value="kn">Kannada</option>
            <option value="ko">Korean</option>
            <option value="lt">Lithuanian</option>
            <option value="lv">Latvian</option>
            <option value="ml">Malayalam</option>
            <option value="mr">Marathi</option>
            <option value="nl">Dutch</option>
            <option value="no">Norwegian</option>
            <option value="or">Oriya</option>
            <option value="pl">Polish</option>
            <option value="pt">Portuguese</option>
            <option value="ko">Korean</option>
            <option value="rm">Romansch</option>
            <option value="ro">Romanian</option>
            <option value="ru">Russian</option>
            <option value="sk">Slovak</option>
            <option value="sl">Slovenian</option>
            <option value="sr">Serbian</option>
            <option value="sv">Swedish</option>
            <option value="tl">Tagalog</option>
            <option value="ta">Tamil</option>
            <option value="te">Telugu</option>
            <option value="th">Thai</option>
            <option value="tr">Turkish</option>
            <option value="uk">Ukrainian</option>
            <option value="vi">Vietnamese</option>
            <option value="zh-CN">Chinese (simplified)</option>
            <option value="zh-TW">Chinese (traditional)</option>
          </select>
        </div>
        <input type="submit" class="btn btn-primary" value="Translate &amp; Download CSV" />
      </form>
    </div>
    <?!= include('_script'); ?>
  </body>
</html>