Translate selected strings in WordPress. Originally from http://www.viper007bond.com/2011/07/13/changing-core-wordpress-strings/

WP-Translate-Strings

// Translate theme strings
function mytranslate_filter_gettext( $translated, $original, $domain ) {

	// Array of original strings and what they should be replaced with
	$strings = array(
		'Contribute Now' => 'Donate Now',
	);

	if ( isset( $strings[$original] ) ) {
		$translations = &get_translations_for_domain( $domain );
		$translated = $translations->translate( $strings[$original] );
	}

	return $translated;
}

add_filter( 'gettext', 'mytranslate_filter_gettext', 10, 3 );