Manual:$wgCategoryCollation

Details
Used when the collation algorithm changes to determine which categorylinks rows need updating.

Currently supports:
 * [default]: make everything uppercase for sorting.
 * [MW 1.17+]: complex, much more multilingual friendly category collation.
 * [MW 1.18+]: sort by binary value of string when stored as UTF-8. Essentially sort by code point.
 * [MW 1.21+]:  with language-specific adjustments, see below.

Since MediaWiki 1.18 extensions can add extra collations via the Collation::factory hook.

Language-specific collations
Since version 1.21 MediaWiki also supports 68 collations designed for specific languages. These are based on  and have the same requirements; they are named , where is one of: af, ast, az, be, be-tarask, bg, br, bs, ca, co, cs, cy, da, de, dsb, el, en, eo, es, et, eu, fi, fo, fr, fur, fy, ga, gd, gl, hr, hsb, hu, is, it, kk, kl, ku, ky, la, lb, lt, lv, mk, mo, mt, nl, no, oc, pl, pt, rm, ro, ru, rup, sco, sk, sl, smn, sq, sr, sv, tk, tl, tr, tt, uk, uz, vi. For example, to use a collation for Spanish, one would use the  collation.

Using these collations provides both correct sorting order for given language and proper headings for first letters of article titles.

Getting new collations added
There are two parts to having a new language supported:
 * It being supported by the ICU library (the list of language codes it supports is available at ).
 * It being additionally supported by MediaWiki itself (this basically requires listing the additional characters, or character groups, that are considered separate letters in the given language, in addition to the basic alphabet) – the always up-to-date list of currently supported ones is available at ).

It might also be the case that the default ICU ordering ('uca-default' collation) orders the titles correctly, but does not correctly separate the letters – it can be used for the first step in that case. Sometimes the letter ordering of a different language might fit yours, if they are related – a custom collation can sometimes be provided in such case (there is one for Sorani Kurdish / Central Kurdish language ('ckb') already, called  ).