Let us start with the headline of your question:
Are questions about explaining some concept of computer science on-topic?
That depends heavily on the question. Software Engineering is more a craft than a science. Surely software engineers make use of achievements in computer science, as lots of other crafts and engineering disciplines use achievements from other sciences, but that does not make any CS topic an SWE topic, or vice versa. Of course, one can ask on this site how to apply a certain CS concept in a specific SWE situation, ideally a real-world situation, not something hypothetical.
Now let us compare this title to your question here:
I want to know what are the differences between immutable and readonly
To my understanding, "immutable" and "readonly" are programming terms with some loose definitions. Still there is no canonical definition which holds to any scientific standard - which makes them non-CS terms. So yes, a question about them is probably be a better fit for Softwarengineering.SE than for ComputerScience.SE.
However, if you decide to ask such a question, you better lower your expectations on getting a strict, clear-cut list of differences.
There is already a question asking about this on this site: Immutable vs Read-only vs Unmodifiable naming difference. However it (1) has a java tag, and (2) is closed for being opinion-based
Though the question had a good title, in the question's body, the asker made the error of putting focus on how to name their class, which is clearly opinionated.
But instead of looking for the comparison of two terms which you did not understand in full, why not start with one term first? Here is a question with an answer* explaining immutability:
What is an immutable object anyway? *(disclaimer: written by me)
After reading that Q&A, maybe half of your question about "immutability vs. readonly" was already answered, and you may consider to ask differently now?
Let me say a word on this:
I want to make sure they are the same regardless of the language
Sorry, but when you don't know the differences between such two terms, what makes you sure the differences are not language-specific? (In fact, I am sure the term "readonly" is highly language specific, since there are programming languages where
readonly is a key word). So instead of presenting your bias on how you expect a certain answer to look like, better stay open for answers which may surprise you. That can enhance the perception of a question by our community a lot.
I also see that we already discussed about this: Should algorithm and data structure concepts still be on-topic for Programmers now that Computer Science has launched?. However it's 11 year old, and one of the allowed topic it says is "algorithm and data structure concepts", which is not listed on the current accepted topic list anymore. So I think we may want to revisit this issue.
I think this is a good idea to discuss this matter and may be worth a question on its own. The question cited was from the time when SWE.SE was still named "Programmers" and the site had a much broader focus. Today, whenever a question is algorithmic in nature, I am under the impression some usual suspects of our community pop up and downvote and close vote it quite thoughtlessly, without giving any further explanation. My opinion here is that we should keep such questions in case they have a practical real-world background, and are not just asked out of a scientifical research context.