Ok, then I look on it:
1. Tax
add this row to lang file:
TAX %S="{translation with %s}"
{translation with %s} - use %s and this part of translation will be replaced by string from system.
2. Item Price
ITEM PRICE="{translation}" should work, tested and it is ok on my site
3. no expiration
open file: administrator/components/com_booking/helpers/booking.php
around line 2378 should be this row:
$ret = "no expiration";
modify it to:
$ret = JText::_("no expiration");
look on other rows like 2380:
$ret = "instant expiration for online payment";
you can modify it to:
$ret = JText::_("instant expiration for online payment");
and line 2395 with:
$ret = self::duration($cancelTime).' before booked event';
you can modify it to:
$ret = self::duration($cancelTime).JText::_(' before booked event');
If it will be in function JText::_("{text}");, it will be translated by your lang file. First text before '=' in lang file have to be uppercased.
4. Reservation Contact
RESERVATION CONTACT="{translation}" should work for you, the last solution should be correct. There is already JText::_('Reservation Contact'); and this translation works on my site.