There is no special sort order for languages yet, you can override this, when adding to the Common.js:

mw.config.set( 'tableSorterCollation', {'Č':'C', ...} );

Ok, but this should be solved globally, for all users by default without having to do an extra setting (via common.js or whatever else).

Doing this for all languages isn't easy at all: For example in German "Ä" is considered as a variant of "A", so it should be sorted at the beginning, while in Swedish it is an own character that comes after the "Z".

I guess we need to specify the default sorting for every language separately.

In T47071#517666, @Schnark wrote:

Doing this for all languages isn't easy at all: For example in German "Ä" is considered as a variant of "A", so it should be sorted at the beginning, while in Swedish it is an own character that comes after the "Z".

Well, but sorting is basics of internationalization... unfortunately this was forgotten here.

Not sure what needs to be done here, but... We use and have set Unicode collation for Categories (uca-cs for Czech Wikipedia). This is already configured for majority of Wikipedias. Why don't we use the same here?