A good design language is certainly more than just a theme. An arrow is a visual design to indicate direction. It's almost universally recognized. It's a sign.

A good design language is certainly more than just a theme. An arrow is a visual design to indicate direction. It's almost universally recognized. It's a sign. Just like a word. If iconography can be seen as a form of barebones language, what else can we fit in the concept? Perhaps colour? Red for STOP, or urgency. Green for go. If shapes and colours, why not relative sizes? Big for important things, small for detailed things. These are the kinds of concerns that pertain to the construction of design languages. You can see how they can be unifying.