Hi,
Falang proも利用しており、翻訳できる単語は英語から日本語にしています。
Then, in the label of the custom field address_VAT you should leave it to "VAT number" and then translate it in Japanese. It seems you've double translated the field label or something.
英語版での決済にて消費税の単語が日本語
That's normal. It comes from the tax namekey you've configured. Go in the menu System>Taxes and click on the "manage tax rates" button. There, you'll see the list of tax rates you've configured. What you've entered for the namekey is what will be used during the checkout. So I would recommend creating a new rate with the name you want in english as the namekey.
Then, you can translate it with a translation override under the Languages tab of the HikaShop configuration.
For example, if you use the text "Sales tax" in English for your tax rate, you can then add the translation override like:
SALES_TAX="消費税"