[KevinH]

Hi,
My guess is that that the Japanese language uses a different whitespace character than English does. I found this in a google search:

in Japanese, there is another space character, commonly called a 'full-width space'. According to my Mac's Character Viewer utility, this is U+3000 "IDEOGRAPHIC SPACE". This is (usually) what results when a user presses the space bar while typing in Japanese input mode.

So you could try a global find and replace for \u3000 to be a normal space " ", or you could try converting them to a numeric entity to see if that is really what is going on.

& # x 3 0 0 0 ;

In Sigil, you can edit your PreserveEntities Preferences in order to make these show up if that is what they are using.

Hope this helps,

KevinH