OK my default forum setting is english (the default one).
I changed the text on that.

My personal setting is
a) english --- shows up as Japanese
b) japanese --- shows up as Japanese
c) english-uk --- shows up as Japanese

Each time, delecting cache and IE cache between trying to set the changes.

Each setting - the Japanese text is appearing. I only changed the text in the default english generic.php.
I checked the other files now and they are English.

What does the "default" do? That I presume is the one that shows when people are guests and when I am logged in without changing the settings.