The current situation is that Serbian is treated as a single language.

My argument for treating it as two languages, other than consistency with Chinese, is that people who are fluent in one writing system may not be fluid in, or even familiar with, another writing system.  For example, if I write a game in English using Braille as the writing system, most English speakers would not be able to read it.  I don't know enough about Serbian or Interslavic to tell how much of an issue this would be for users of those languages.  Of course this only matter if it's possible to filter by language.

(Note: Chinese (simplified) and Chinese (traditional) literally are exactly the same language, differing only by writing system.  It has nothing to do with dialect.  All Chinese dialects that can be written with simplified characters can also be written with traditional characters and vice versa.)