Why can’t some special characters be used for CD-Text?
Unfortunately this is a general problem of the CD-Text standard that was defined by Sony.
CD-Text uses the ISO 8859-1 character encoding, which includes the most common characters used in western european languages. Unfortunately not all languages are covered completely. Especially some characters used in eastern european languages are missing.
Alternativaly the CD-Text standard also allows for Japanese, which is stored using the so called 2-byte Shift-JIS character encoding. This encoding does not include the European special characters, so a mix of both is not possible. To enter CD-Text in Japanese, you need to switch Settings->CD-Text Language to Japanese (CD-Burn & DDP Version 1.6.7 or later).