Case insensitive sorting doesn't work for non ascii characters

Question

Make a list of these words:

Če
Čeprav
Človeški
čaka
čakal
čakalne
čakati
časih
časovni
času
če

Select all: ggVG and sort them with case insensitive: :sort i

Če
Čeprav
Človeški
čaka
čakal
čakalne
čakati
časih
časovni
času
če

You may have already detected a problem... But let's compare it with lowering case and then sorting them:

First, convert it to lower case: ggVGgu. and then sort them: :sort. You get:

čaka
čakal
čakalne
čakati
časih
časovni
času
če
če <-- DIFFERENCE
čeprav <-- DIFFERENCE
človeški <-- DIFFERENCE

I was expecting no differences between the 2 methods, so I don't get why I have this inconsistency?

Answer 1

Since Vim release 8.2.0988, Vim supports sorting by the collation. That means, it uses your currently configured locale ($LC_COLLATE). Using the collation order also means, there are specific rules for sorting multi-byte characters for your configured locale.

If it is set, you can see by the output of :lang or using :echo v:collate.

So by setting your locale correctly you can sort using the :sort l parameter. Usually, sorting by the collation is also case insensitive, although that does not always have to be.