Is it possible to make/add custom language into the OS? (sry for poor english)
Not sure what you mean. The later versions of OPENSTEP supported objective C, C++, and I think some Java, depending on what you were doing. Those were languages supported by the libraries/kits.
Other languages work as well. I think there may have been LISP, straight C, and a few other languages you could use/compile on the machine, but dont think they easily integrated into the NeXT development environment, so they were more for programming your own independent things, eg like a command line/posix app.
could the OP have been asking about human languages, not computer languages?
I also think OP is talking about localizations.
Oh, sorry, my bad. Yes, there is also language localization stuff in OPENSTEP 4.2 if I recall. Basically the initial version of what you see in macOS these days. Most language localization was pretty easy, but if I recall was a little different for Japanese back then (and you needed a specific version of the OS to deal with Japanese).