In my opinion, the language that comes closest to K's functionality and is also understandable by mere mortals is K itself. It is obviously extremely close to K's functionality and is a very simple language, the only reason it doesn't seem simple is that most people are used to verbose languages. A couple of days of practice is enough to make K readable, in my experience.

Also, I am constantly amazed at how concise K is, easily rivaling not only conventional languages but also much larger array languages like APL or J. Arthur Whitney's taste in selecting primitives is out of this world.

I'm extremely biased in recommending it, but Lil is semantically very similar to Q, entirely free, and intended to be beginner-friendly: https://beyondloom.com/tools/trylil.html

It's not as powerful or concise as K, but it gives you some of the flavor of an array language tucked inside what resembles an ordinary imperative/functional scripting language.

I recommend checking out uiua.org for fun. The docs are well written and the concepts, while foreign to most, are ultimately accessible and interesting.

k and uiua are in different branches of the APL family.