Locale Switch is an Omeka S module that provides a view helper for allowing users to change the locale. The chosen locale is then stored inside user's session.
Enable the module then add this into your theme:
<?php
echo $this->localeSwitch([
'en' => $this->translate('English'),
'fr' => $this->translate('French'),
/* ... */
]);
?>
You can customize the HTML output of the view helper by overriding
locale-switch/helper/locale-switch.phtml