How was multiprogramming brought into computer languages
If you're really interested in all that's needed, your best choice would be to look at the standardisation process of C++11 by the ISO Working Group on C++ (WG21). All the relevant papers are available online.

Keywords are not important at all. In fact, C++ defined just one new keyword (thread_local) and that's mostly for convenience. 

What is important is the memory model. Programs work by altering state, which logically exists in memory. With multi-programming, there are concurrent changes to the state of a program. This requires a reasonably precise definition of said state. To make things more complex, you want a definition of state that is compatible not just with the abstract language definition, but which is also implementable on real hardware. That gets you into problems like registers and  caches (state outside ordinary memory), the exact details of which vary significantly between real hardware architectures.

A key insight here is the sequenced concept. Two C++ operations can be sequenced, which means they cannot operate at the same time. This means they are allowed to use the same data in memory without risk. A special kind of sequencing is sequenced before, which means one operations happens before the other. This means a write sequenced before a read guarantees that the read sees the value of the write. 

With C++11, there are library calls defined which can guarantee such sequencing across different threads of execution.

The comments of @rcgldr suggest that function calls like WaitForSingleObject alone are sufficient, but this isn't actually the case. You need an associated memory model. That's why you have Win32 macro's like InterlockedIncrement - these macro's provide the mapping between the language memory model and the hardware memory model. There's no Read(DWORD*) macro, though - the language memory model is that aligned DWORD reads are atomic, and it's implicitly assumed that the compiler can efficiently map every read to atomic hardware instructions.