If your moodle is set to ja_uft, or it contains ja_uft contents, then changing to the English language pack can garble all the conents in the course, unless you keep manually changing the page encoding to "UNICODE(UTF8)" in the browser.
Attached is a micro-language pack which displays the Moodle GUI (menus, side bar etc) in English but does not garble the Japanese, because all it does is specify that the page encoding be in unicode (ascii English displays fine in UNICODE).
It is based on the en_jp language pack which did the same for the EUC-JP encoding.
You must have the conventional english language pack installed as well, but you do not need to give people the option of using the conventional English language pack, if you disable it in the lang list site setting.
Tim