Rated 4 out of 5 stars

But can I download it in English - or I must translate it?

This review is for a previous version of the add-on (1.1.2). 

So sorry, I have not prepared explanation of English about this add-on yet.
Please wait a moment.
Actually latest version has jp and en-us locale. And If your thunderbird locale setting in en-us, you can see in English.
I will be going to prepare two or more languages in the future.

Thank you.