from https://bugs.launchpad.net/bugs/39626
"In case of Japanese input, candidates look up table should be vertical by default.
This is because Japanese candidates table tend to be longer Chinese and Korean's one, so it's too wide to look up using horizontal table. Vertical lookup table is used on Windows and Mac Japanese Edition.
In Chinese and Korean case, it's good to set horizontal look up by default, so this is conflict issue between Japanese and other locales. I guess it's fit to language-selector."
and
"see [scim-setup] - [Panel] - [GTK] - [Input window] - [Vertical lookup table]"
The Tracker is assigned to "Feature Requests."