I have software originally developed 20 years ago in Visual C++ using MFC without UNICODE. Currently strings are held either in char[] or CString, and it works on English and Japanese Windows PCs until Japanese characters are used, as these tend to get converted to strange characters or empty boxes.
Setting UNICODE is presumably the way forward but will require a massive code change, whereas quite a lot seems to work simply by setting System Locale to Japan (in “Window’s Language for non-Unicode programs” setting). I have no idea how Windows does this, but some Japanese character things now work on my English Windows PC, e.g. I can open and save Japanese filenames with no code changes. And in Japan they set System Locale to English and again much works, but not everything.
I get the impression the problems are due to using a font that doesn’t include Japanese characters. Currently I am using Arial / MS Sans Serif and charset set to ANSI_CHARSET or DEFAULT_CHARSET. Is there a different font I should be using, or can I extend these fonts to include Japanese characters? Or am I barking up the wrong tree in which case what do I do next? Am very new to all this unfortunately …
That's a common question (OK I guess not so common any more in 2015, as MBCS programs luckily are a dying breed - I still maintain several though...)
Either way, I'm afraid that, depending on your definition of 'working', to get this working you'll have to bite the bullet and convert to a Unicode build. If you can't make a business case for that, then you'll have to set the right locale (well, worse, have the user set the 'right' one) and test what works and what doesn't, and ask more specific questions on what doesn't.
If your goal is to make one application that correctly displays strings in various encodings in the 'right' way regardless of the locale settings on the computer, and compatible with every input data set / database content without the user having to be aware of encoding issues, then you're out of luck with an MBCS build.
The font missing characters is most likely not the problem. Before you go any further and/or ask further questions, you should read http://www.joelonsoftware.com/articles/Unicode.html, read it again, sleep on it, read it again, explain to somebody else what the relationship is between 'encoding', 'locale', 'character set', 'font' and 'Unicode code point', because only after you can do that, you can decide on how to progress with your application. Sorry, it's not what you want to hear, but it's the reality if you've been tasked with handling internationalization.