Manual:$wgCategoryCollation

Details
A version indicator for collations that will be stored in cl_collation for all new rows.

Used when the collation algorithm changes: a script checks for all rows where cl_collation != $wgCategoryCollation and regenerates cl_sortkey based on the page name and cl_sortkey_prefix.

Currently supports  (make everything uppercase for sorting) and   (complex, much more multi-lingual friendly category collation) collations.

In MediaWiki 1.19 it can also take a class name as its value, where it will use that class as the Collation class.