Manual:$wgCategoryCollation

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

Currently supports  (make everything uppercase for sorting),   [in 1.18+] (sort by binary value of string when stored as utf-8. Essentially sort by code point), and   (complex, much more multi-lingual friendly category collation) collations.

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

Language-specific collations
After is merged (probably in time for 1.21) MediaWiki will also support 67 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, 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.