Thank you, spg Scott, but I am informed by my daughter that she was not prompted for a language setting during the installation. I was on the phone with her during most of the steps for the installation and she also didn't make mention of any request about a language during that time.
Unfortunately, she didn't inform me right away that the language was suddenly in Japanese. And I can't seem to raise her on the phone as of a moment ago, so I'll ask tomorrow if she can remember when she first saw the Japanese language showing. But like I have noted, she started from an English-language page and didn't select Japanese from any menu.
DavidR, thank you for your input, but I have twice installed avast on computers that have Japanese-language Operating Systems and in neither case did the installation process include an auto change to Japanese from the English of the page I started from for downloading.
I can't be sure about the most recent installation on the Epson. I can look into my logs and see if the language pack installation for the Vista Ultimate switch to English was before or after I installed Avast. But I already know about the two previous installations on computers with the Japanese OS. This is the first experience I have of an auto language change.
And I wish to be sure I understand that the domain name "avast.co.jp" is owned and run by the avast folks there in Europe, right? I know it seems I am harping a lot on what may seem like a trivial detail, but if you were familiar with the difference between Japan Microsoft and USA Microsoft/Australia Microsoft/etc. then you'd understand my concern. The Japan Microsoft is a whole different animal. A very, very mean one. A very, very not nice one.