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.