same :D :D :D

Read Clarifications part of the description to know how the clients go to the tills.

Totally agree. The kata doesn't say that the fastest customers check out first.

[...] it works perfectly in my editor.

That doesn't make your solution correct. It's very likely that you're using biased and limited inputs, which give faulty solutions (like yours) the appearance that they are correct.

What my function returns and what is expected are exactly the same.

That is false:

Z-Pp-Ggg-Llll-Nnnnn-Rrrrrr-Xxxxxxx-Qqqqqqqq-Eeeeeeeee-Nnnnn-Uuuuuuuuuuu      // actual
Z-Pp-Ggg-Llll-Nnnnn-Rrrrrr-Xxxxxxx-Qqqqqqqq-Eeeeeeeee-Nnnnnnnnnn-Uuuuuuuuuuu // expected

Moreover, sometimes when I submit, it passes 100 tests and fails 1, other times, it fails them all. I don't understand.

Your solution contains a very common beginner's mistake. I recommend taking a deeper look at the failed test case to figure out what is going wrong.

Your code is not case-insensitive. So it doesn't work there either, you just didn't check it with the same input values.

till 1 will serve 2 then 10

till 2 - 3 then 2

total time - 12

You're not testing it properly there.

It should default to English if the language is not in the database, or in the event of an invalid input.

I tried coming up with some unique ways to call 'hello world!'. Had to settle for the easiest method though :P