This is another one of those questions where terminology (or how they are inconsistently applied) can make it really difficult to understand, so again in my answers to these types of questions I'll try to stick to the concepts/ideas and leave others to work out the semantics of it all.
Generally speaking, a style guide and/or a pattern library (you could also argue that a development framework fits into the same category but for developers) at the minimum should give you all the components to put together actual designs to a particular specification, in the same way that you can go to the IKEA store to grab lots of things packed in boxes and put together a piece of IKEA furniture.
But as we know, sometimes it is not that straight forward to work out how to put things as they look like in the picture, so good style guides or pattern libraries will include a set of instructions to give you some idea of how to put the components together.
Sometimes in addition to the instructions, the person who has come up with the concept for the furniture also wants to provide some background or explanation as to why things have been put together in this way, and what makes it different to something else. Most of the design frameworks (Material Design, Bootstrap, Atomic) that I have seen seem to provide all three components, and in a way they are probably equivalent to the design systems that you refer to.
So what we have essentially is the design philosophy, the components and the instructions for how to assembly them. I am making the assumption that a design system can be used to describe something that incorporates all those elements, while a design language is a set of instructions for how to use the components, and the style guide and pattern library gives you the actual pieces to put together.
Of course, I could be completely wrong about the terminology, but hopefully the general idea for what you need in an effective design toolkit to develop a consistent user experience (in the form of user interfaces) is close to the mark.