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 ( 'description' === strtolower( $text ) ) {
			return 'Beschrijving';
		}
		// add more conditions, changing as many strings as you'd like
	}
	return $translations;
}
add_filter( 'gettext', 'si_change_strings', 10, 3 );

function si_change_strings( $translations, $text, $domain ) {
	if ( 'sprout-invoices' === $domain ) {
		// Change "Description"
		if ( 'print to pdf' === strtolower( $text ) ) {
			return 'Download Invoice';
		}
		// add more conditions, changing as many strings as you'd like
	}
	return $translations;
}
add_filter( 'gettext', 'si_change_strings', 10, 3 );