Generally, the software tries to select a font that supports the appropriate charcters. If the default system font does not support a certain character set, the software tries to choose another font from among the other available fonts (from those that are part of the original software, as well as from the custom fonts). Unfortunately, this does not work properly in some cases. In those cases, the software "believes" that a certain font supports the needed characters, even though it does not. To work around this problem, you can delete all fonts that the software selects instead of the one you wish. Have a look at
nickel GUI not displaying certain characters.
Note: In order to install custom fonts, create a directory "fonts" and put your font files into it.