WordPress at a glance
action is not described

switch_locale action-hook . WP 4.7.0

Fires when the locale is switched.

Usage

add_action( 'switch_locale', 'action_function_name_64' );
function action_function_name_64( $locale ){
	// action...
}
$locale(string)
The new locale.

Where the hook is called

WP_Locale_Switcher::switch_to_locale()
wp-includes/class-wp-locale-switcher.php 90
do_action( 'switch_locale', $locale );

Where the hook is used (in WP core)

Does not used.