Email notification cannot be modified

I want to translate my email notifications. So I edited booking_class.php $res_info like this:

$res_info = "<table border='1' style='border-collapse:collapse;min-width: 360px;'>
<caption style='text-align:left;'>DetailsInMyLanguage</caption>
<tr><td style='padding: 1px 7px;'>Reservation datesInMyLanguage</td><td style='padding: 1px 7px;'>".$res_day."</td></tr>
.....

The modifications work perfectly when the booking request has been sent.

But when I Approve/Cancel/Reject the booking request the plugin still sends English letters.

Any idea?

https://ww.wp.xz.cn/plugins/booking-calendar/