This is the code in wp-includes/formatting.php that supplies the separators:
$l = apply_filters( 'wp_sprintf_l', array(
/* translators: used to join items in a list with more than 2 items */
'between' => sprintf( __('%s, %s'), '', '' ),
/* translators: used to join last two items in a list with more than 2 times */
'between_last_two' => sprintf( __('%s, and %s'), '', '' ),
/* translators: used to join items in a list with only 2 items */
'between_only_two' => sprintf( __('%s and %s'), '', '' ),
) );
As you can see, the strings ‘%s, and %s’, and ‘%s and %s’ are passed through the standard translating function __().
That means you can add those strings to the standard translation tables for your theme. Here is an article that may help explain how to do that: https://code.tutsplus.com/tutorials/translating-your-theme–wp-25014
Another way to do this is to take advantage of the filter that is used by apply_filters(). Add a function like this to your functions.php file:
function translate_list_separators( $separators ) {
$separators['between_last_two'] = ', plus ';
$separators['between_only_two'] = ' plus ';
return $separators;
}
add_filter('wp_sprintf_l', 'translate_list_separators');
Substitute your own translations for ‘, plus ‘, and ‘ plus ‘.
