According to Roan in T32287: Implement uca-fa collation comment 9, actually enabling the uca-default collation stuff that was "fixed" for T2164: Support collation by a certain locale (sorting order of characters) is waiting on an Ubuntu upgrade on the apache cluster (T31915: Upgrade the WMF-cluster >= Ubuntu 10.04?).
There are a few bugs which it looks like should be resolved (for Categories at least) by enabling this -- eg T32287: Implement uca-fa collation (Farsi sorting problems); others require further work (T31788: Swedish-language wikis should use Swedish-locale sorting (ie. ÅÄÖ should sort correctly) needs a Swedish-specific collation setting).
See Also T32673: Implement central locale-specific, or tailored, sorting framework (tracking)