Yes, they're not the same for every language, check Python or Ruby for instance. OP used Python, and those sample tests are exhaustive. C# and others as you wrote, are rather barebones and should replicate Python's ones. Also, not all languages have None, that's why even with missing sample tests, you can assume to use "" in those.

@Chrono79 Where does one view the sample tests?

If you mean in the bottom code editor when doing the kata it gives three examples, but none of them feature one that requires an empty string.

Is there a difference between languages? I'm attempting C#, there's three tests that are rather barebones.

The description does not mention that, when the user reaches rank 8, progress must be 0. It says "A user cannot progress beyond rank 8", which I interpreted as it can not rank above 8, but I think it is unclead if progress should be still counted or not.

That depends on the language, the whole paragraph is this:

If a string contains all repeating characters, it should return an empty string ("") or None -- see sample tests.

So, you should see the sample tests about which one you should return.

Hi. The kata says that if all the characters are repeated the program should return '' or None. However, returning None fails various tests. The description should be changed.

You could just check if both division remainders and return False in that case, since the only True condition is when they are different.