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A customer calls and tells you that your software does not display a Chinese, Russian, French, or whatever character in the user interface of that great new software you released.
What do you do?
Several things, of course. You ask for that screenshot which most initial inquires never have attached. You may also have to ask whether they see question marks or squares or something else. You look up your source control, you check a running instance of your software, and so forth.
If your customer is using Java, send them the .war file in this font diagnosis package. Ask them to deploy it on the machine that shows the display problem. They can then enter any characters they want to check for, and the application will list all fonts present of this machine that can display the characters.
Not only is this fun, but it will give you a much better idea what to do next. I have seen this tool return no font at all, which meant in this instance that the operating system did not come with a font that you would normally expect. In other words, someone shipped a box to China without support for Chinese fonts. Bummer.
In another case, the machine listed several fonts capable of displaying the required text. So, someone had written bad code.