Nowadays, we have confronted the problem about sorting pages in categories;
they are grouped only very first letter.
But the nature of Thai language has some front-vowels -- เ แ โ ใ ไ -- before consonants
such as "เดียว", it should be ordered under "ด", should not be under "เ".

Additionally, tone marks are treated as primary sortkeys
that causes the ordering messes up,
"ด้วย" comes after "ดิน" which is incorrect for example.

Some time ago, a makeshift is made for the problem by using DEFAULTSORT.
But consequently, it must be added everytime we created them;
it labours our users.

Since 1.17, there is a correct way to sort these pages:
$wgCategoryCollation is born to solve the problem.

UCA has pretty good collation for Thai characters (and other alphabets too):
the front-vowels are sorted after their consonants,
the tone marks are sorted as secondary level.
http://www.unicode.org/charts/uca/

I must request adding this to Thai Wikipedia (and also other Thai sister projects):

$wgCategoryCollation = 'uca-default';

This will solve the long-time issue that we have experienced.
(Please don't forget to run update script.)

I have tried in my localhost and it works well.
"ด้วย"→"ดิน"→"เดียว" are correctly sorted under "ด" without DEFAULTSORT.

Version: wmf-deployment
Severity: enhancement