How to translate tax rates ?

Hi,

There is no system to translate the tax rates.
You would have to edit all the places where tax_namekey is displayed and add a JText::_() on it in order to be able to add translation overrides on the namekey of the rate.
It's not a problem if your a developer.
For example, for the checkout cart display, you would edit the file "cart" of the view "checkout" via the menu Display>Views and change the line:
<?php echo $tax->tax_namekey; ?>
to:
<?php echo JText::_($tax->tax_namekey); ?>

Then, if your namekey is VAT, you could just add such translation override:
VAT="TVA"