[jonnyl]

Assuming you're using a Kobo with KFMon, that's easy. Connect your Kobo via USB under Nickel, and then:

1. Copy your entire .adds/koreader folder to a new folder, like .adds/koreader-jp or whatever you'd like to use for your desired language and only keep the dictionaries you need in its data/dict subfolder.
2. In the base folder, copy the koreader.png icon to koreader-jp.png and edit it so you can easily recognize it (e.g., overlay a "JP" text with an image editor).
3. In the .adds/kfmon/config folder, copy your koreader.ini to koreader-jp.ini, then open it and edit the filename to the icon (koreader-jp.png), the path to the launch script (/mnt/onboard/.adds/koreader-jp/koreader.sh), the label (e.g., KOReader JP)
4. Eject your device

Your Kobo UI (Nickel) will then show the new icon as a new 'book' that launches the new KOReader.

I think that's all. It would be cleanest if you copied the folder after a fresh initial installation (from scratch). It also works with an existing installation, but you may want to delete some files in the new copy, such as crash.log, cache files, history.lua, etc., that don't pertain to your additional installation.