Then, in my opinion, if the order is important to the solution (which it obviously is) this should be stated in the description of the task. At least as a hint.
The diff isn't really helping finding the source of the problem when the order isn't correct (just too many results to really get a sense of what might be the problem).
Even considering that the order isn't defined when not specified in the statement and that then the source of the error isn't the script but the server itself?!

The order of the rows seems to be important. My first solution was technically correct, but had the EU sales first and then US sales, which resulted in the solution being wrong. Is there a way to make the tests order-independent?

In your ternary in the else part, you are missing the .reverse().toString() on the StringBuilder instance.

Still, returning a function from within a function, also sometimes known as a delegate (like in C#) is a basic language feature.
8kyu really might be too difficult, but i think it shouldn't be any lower (or is it higher?) then 7kyu too.

Thank you for the code for the header. It seems that it is contained correctly in the discription but while writing my translation, it didn't display correctly in the preview.
We'll see if it works once the translation is accepted (once you can see it, that is).

C# version kumited.
Unfortunately, it looks like my changed description destroyed the top part (info about series) and I have no idea how to fix it.

Remark: I made single tests instead of only two tests with multiple asserts like the Java version, since that's how it "should be done" in C#/NUnit.
Also, i wrapped every test body in Try/Catch with an Assert.Fail() in the Catch because codewars just ignores tests containing exceptions, sometimes resulting in "valid" solutions that just contain a simple throw new Exception();.

C# version kumited
some more test cases, but no random test case
also I added a rule (for now only for C#) for inputs smaller 1

C# version kumited
i also made single tests instead of using one big test with multiple asserts
and i added one more test to the final tests

C# translation kumited.

(Description modified)

I had to omit one test case (based on Ruby test cases) with the [nil] Array.

The requirement isn't asking for the time (in days) he needs to read the book but the integer representation of the day of week when he finishes the book (finished on a Monday (1) and so on)

So, reading a 2 page book with 1 page each Monday, the result is 1 because he finishes the book on a monday.