Usually what you want to do is use the Drupal language switcher and choose the option in (civicrm/admin/setting/localization?reset=1) that tells CiviCRM to follow the language that Drupal is using ("inherit CMS language").
To work properly, that option requires you to fill out a couple of lines in the civicrm.settings.php that are commented out by default, but which tells civicrm how to map the Drupal language settings (which are just two letter language codes) over to CiviCRM language codes (which are two letter followed by a region code).
Here's an example from a Canadian bilingual site of mine:
# grep LANG civicrm.settings.php
// define('CIVICRM_LANGUAGE_MAPPING_ES', 'es_MX');
// define('CIVICRM_LANGUAGE_MAPPING_PT', 'pt_BR');
// define('CIVICRM_LANGUAGE_MAPPING_ZH', 'zh_TW');