I fixed most of your points ✅. I removed the return type completely, because if the return type would be a double[] then stringArray.collect { it as double } would be a valid solution without converting the list back into an array.

Thank you!

Good kata Товарищ! USSR гордиться тобой! "Учиться, учиться и еще раз учиться!", как завещал товарищ Ленин(Not a Jonh Lennon XD)

Considering all translations are good + this kata seems to be unique, I've given this kata 7 kyu :)

Ohh ok, I see what you meant. Approved your fork ✅

That's not quite what I meant. I created a fork for your future reference.

Groovy Translation

Replaced test.describe with test.it ✅

missing it block in random tests.

Groovy Translation

Implemented your remarks ✅
Even though there is a reference solution now, I decided to keep it in for simplicity's sake

Style remarks:

• the case_ = randint(1, 2) is generally unnecessary. Just generate X cyrillic letters, Y non-cyrillic letters, and shuffle them together. This also saves you the trouble of having a reference solution.
• sol_495banana541 is not necessary, and a "normal" name, like refsol or reference_solution or something like that should be good?

I think that the point 3 would apply also to is_russian function name and other names in the code.

Fixed all of your points and thanks for having a look over my kata ✅

For the first kata that's actually not bad, but there are some advices regarding the testing code:

1. Consider moving random tests to @test.describe instead of @test.it within fixed tests section — that may confuse some people like me;
2. letter = chr(randint(0, chr_MAX_PARAM)) # Not including \0 just in case — this can actually spawn \0 as one of the tests, because randint takes the random number within the parameters included;
3. Please move your reference solution inside of Random Tests section — for more information about the authoring consider referring to Codewars Docs of authoring for Python and/or katas selected as authoring examples;

Also I'd like to propose changing the title to 'Cyrillic letters', since the Russian language does not have all the letters of Cyrillic block ;)