Hi there,
It took me a little while to figure it out.
In fact, it has nothing to do with Windows... the locale settings are directly applied within the playonmac.app application (so, at the MacOS level).
Playonmac.app gets the locale settings automatically from the MacOS language used. You can see how by checking into the packet content of that application. To do so, you need to browse to playonmac.app/Contents/MacOS and edit the playonmac script there.
On the version 4.4.3, the locale part in that script is :
export RLANG="$CurrentLocale"
export LANG="$RLANG.UTF-8"
export LC_ALL="$LANG"
Now... I understand though that it may be hard to switch the OS language to simply play to a specially localized game .
I have used the following solution/workaround to set the locale I wanted :
1- Duplicate the Playonmac.app (and rename it to identify that it will be used for games using japanese locales).
2- Go and edit the playonmac script (using any editor like Textedit, vi...). Change the locale part (mentioned above) with the following :
export RLANG="$CurrentLocale"
export LANG="ja_JP.UTF-8"
export LC_ALL="$LANG"
Save the change.
3- You can launch that modified version of playonmac.app to run your favorite japanese's windows applications. As a plus, you may even specifically name the application shortcuts within playonmac for the applications/games that require the japanese locales.
Last rermark : this workaround can be used for any other locales (if you were looking of thinking else than japanese). You can check in a terminal all the available locales on your mac by using the following command :
locale -a
Cheers.