Please add following code in your theme’s (child theme’s if any) functions.php

add_filter( 'fep_directory_output', function( $directory ){
	$checkbox = '<label><input type="checkbox" class="fep-cus-select-all" />'.__('Select all').'</label>';
	$checkbox .= '<script type="text/javascript"> jQuery(document).ready(function(){';
	$checkbox .= 'jQuery("#fep-table .fep-cus-select-all").change(function(){';
	$checkbox .= 'jQuery( \'#fep-table input[type="checkbox"]\' ).prop("checked", this.checked);';
	$checkbox .= '})}); </script>';
	
	return str_replace( '<div id="fep-table" class="fep-table fep-odd-even">', '<div id="fep-table" class="fep-table fep-odd-even">' . $checkbox, $directory);
});

It will show a checkbox in “Directory” to select all users from directory. Then from dropdown select “Send message”.

Please wait for next version. It will be added in plugin, so you do not need any custom code for this. Next version will be released next week.

When release next version, you will be able to translate like any other string. To translate above code just change Select all to your language.