Translating Strings in Sprout Invoices

functions.php

<?php // don't add this line, since your functions.php file already starts with it.

/**
 * Example filter will translate/change the string "Description".
 * "Description" can be changed to whatever string you'd like to translate/change
 * just make sure to change the function name so you don't have any conflicts.
 */
function si_change_strings( $translations, $text, $domain ) {
	if ( 'sprout-invoices' === $domain ) {
		// Change "Description"
		if ( '<b>Payment Due before %s:</b> %s' === $text ) {
			return '<b>Payment Due %s:</b> %s';
		}
		// add more conditions, changing as many strings as you'd like
	}
	return $translations;
}
add_filter( 'gettext', 'si_change_strings', 10, 3 );