Hi Terence,
After you put the en_ja language folder into the lang folder of moodle, did you do the following steps?
1. Go to Admin >> Configure >> Variables and type en_ja,ja into the "langlist". Or add more languages you wish to show to users in the list. Save.
2. Go to Admin >> Configure >> Variables and set en_ja as the "default language". Save.
3. Go to Admin >> Configure >> Language and set English - EUC-JP (en_ja) as the "Current Language"
I am not sure of the particular order of those steps or if all of them
are absolutely necessary, but after I did all of the above, I noticed
my en_jp then began to work. I now have no mojibake.
But please note, when switched in Japanese mode, I noticed the kanji
for the day of the week did not appear (see attached file). Did
anyone else have this happen?