At a macro level, you learn programming concepts that are largely transferable regardless of language. The syntax may differ, but the core ideas are still the same. This can include things like: data-structures (arrays, objects, modules, hashes), algorithms (searching, sorting), architecture (design patterns, state management) and even performance optimizations (e.g. eager vs lazy evaluation, memoization, caching, lazy-loading etc). These are concepts you’ll use so frequently that knowing them backwards can have a lot of value.
At a micro level, you learn the implementation of those concepts. This can include things like: the language you use (JavaScript, Python, Ruby, etc), the frameworks you use (e.g. React, Angular, Vue etc), the backend you use (e.g. Django, Rails, etc), and the tech stack you use (e.g. Google App Engine, Google Cloud Platform, etc). There involve details that can be valuable to gain expertise in to be effective, but are not always transferable.
Software Engineering – The Soft Parts