In disbelieve, I double-checked my own steps. And finally, I've got it working now. I am embarrassed to say the problem was with me, the instructions were given on the posts, I just didn't get it. Somebody made even the same mistake and said so, and I still didn't notice.
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FAQ section "Why are Japanese characters displayed incorrectly?"
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Forum post "Japanese JIS format"
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Forum post "4.7.2, Unicode and UTF-8"
Here are the steps I took, in my own words, plus comments where and why I got stuck. Please correct me where I am wrong:
* It seems to be a requirement to configure a "document class" to work specifically with Japanese. TextPad needs to know that for particular files it should use a particular fonts, one with Japanese characters of course.
* It's done with the following steps:
* Menu "Configure", choose "Preferences..."
* Look through the tree on the left. Expand "Document classes" (click the cross "+")
* From the tree how you can also choose a specific document class; choose the class you wish to enable for Japanese
* Now also expand that document class in the tree view, this should bring up more settings, including font
* Click front - this finally brings up the settings we are looking for
* Here you have to choose a font that includes Japanese characters. As it is said in other posts, "MS Mincho" is one of them. There are probably others (?). You are basically now assigning that font to this file type/document class, and it will always be used for it, just exactly the same way normal text files use the font "Courier".
* But this is not all, and
the last step is what I have overlooked.
Besides choosing a font, you also have to choose a script. You do this from another combo-box/drop-down-box labeled "Script:" further below. Normally, it's set to "Western". You have to choose "Japanese"! That's probably also a way to check which fonts have Japanese scripts - the combo-box seems only to have other options if the font contains other scripts.
* I guess that's all. Click ok.
* Try copy&pasting some Japanese text from a website (e.g.
http://www.guidetojapanese.org/) into TextPad. If you see proper Japanese characters, you are good. If not, you see ??????. Better also check saving the file and opening it again, to make sure it also works with the encoding.
* I can't remember, but for my Windows Vista I did not have to install additional fonts or go to the region settings. But as I say, I can't remember. I might have done something some time ago when I enabled my browsers for Japanese.
* Be aware that once you picked a font and non-western script, you stuck with it for this file type/document class. It might not even contain some western characters anymore. To me it's not yet clear how you can work in TextPad with Japanese and western text simultaneously, if for example you want to mix Japanese with German in normal text files (.txt) - you are now missing some characters from German.
* Be the way, you could also create a new document class, if you wish, by opening the menu "Configure" and choosing "New document class...". There you can associate the document class with a certain file extension, e.g. "jtxt" as in Japanese-Text-File.
I also like to point out two other things, although I am not sure whether they are relevant or helpful (or even correct):
1) The files are saved to the hard-drive in specific encoding. If you go for "Save as...", you see at the very bottom the box for "Encoding". This is usually set to ANSI. Japanese seems to save ok with ANSI, although I thought it might need to be set to Unicode.
2) When you go to the menu "View", then "Document Properties", you see under tab "Document" the encoding type ("Code set"). Under tab "Font" you have the same settings you saw under the document class properties.