Task: Question Generation
Input (English): NeXTSTEP (also stylized as NeXTstep, NeXTStep, and NEXTSTEP) is a combination of several parts: a Unix operating system based on the Mach kernel, plus source code from BSD Display PostScript and a proprietary windowing engine the Objective-C language and runtime an object-oriented (OO) application layer, including several ''kits'' development tools for the OO layers NeXTSTEP is notable for having been a preeminent implementation of the latter three items.
Output (English): What is the programming language for NeXTSTEP?