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.