Root Cause Analysis:
My best guess is that the theme you're currently using does NOT use the translate function ( __()) for that specific Label.
I see you've mentioned Amasty in the question/post, so I assume you're using an Amasty Theme. If so - You could ask Amasty for a Patch to address that issue.
(In my experience, they'll usually give you a ...
I have done using the plugin. I override the array for tax information.
<plugin disabled="false" name="...
Your problem here is that the original class that calls getFullTaxInfo() extend explicitely Magento\Sales\Model\Order\Pdf\Total\DefaultTotal (see the code below) so it does not load your overrided class.
Since the function you are trying to override is a public function you should try to use a plugin on this function instead. It is a good practice to always ...
You can use the event 'sales_model_service_quote_submit_success'
and add the custom value in quote to check the Quote data, and Apply Condition there, So It Won't Effect the frontend orders.
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:...