use DrupalCoreLanguageLanguageManager;
$standard_languages = LanguageManager::getStandardLanguageList();

foreach($standard_languages as $key => $val){
    if($val[0] == "English"){
        echo $key; //write your code

    }
}

You can do it with this function.

function getCode($find) {
  // Getting all the languages.
  $languages = Drupal::service('language_manager')->getStandardLanguageList();
  $code = NULL;

  foreach ($languages as $code => $names) {
    for($i = 0; $i < count($names); $i++) {
            if ($names[$i] == $find) {
        return $code;
        }
    }
  }
  return $code;
}

// Language to search.
$find = 'French';
// Calling the function.
$code = getCode($find);
// If you have the devel module installed you can use the dpm function.
dpm($code);