OOPs, or Object Orientated Programming is one of the trickiest yet most interesting concepts in the programming/coding world. Accordingly, programming languages are designed to serve the purpose of OOPs.
In OOPs, coding is done by creating objects and classes. An object is an instance of a class which means the class is a description of a set of objects.
There are other languages like python, java, CPP, etc. which support the OOPs and they might sometimes be called Object Orientated Programming Language, but in reality, they are Semi or Partial Object Orientated Language. Since they allow ways other than OOPs. So, they cannot be called Pure Object-Oriented Language. There is this only one language in the world that can be termed a Pure Object-Oriented Language. It is Smalltalk.
Smalltalk is the only pure object-orientated language released in 1972. It was developed at Xerox Alto. This unlike other languages does not support primitive data types like int, char, float, bool, etc. In this language, there is no difference between values that are objects and values which are primitive types.
It has an incredibly rich programming environment as everything is an object. Its simple syntax, closures, lexical scoping, dynamic-typing, and automatic garbage collection make it easy to learn.