Version:           unknown (using KDE 3.5.5, Kubuntu (edgy) 4:3.5.5-0ubuntu3.3)
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.17-11-generic

Scenario:
* log into KDE
* user's locales are set as follow

LANG=pl_PL.UTF-8
LANGUAGE=pl_PL:pl:en_GB:en
LC_CTYPE="pl_PL.UTF-8"
LC_NUMERIC="pl_PL.UTF-8"
LC_TIME="pl_PL.UTF-8"
LC_COLLATE="pl_PL.UTF-8"
LC_MONETARY="pl_PL.UTF-8"
LC_MESSAGES=C
LC_PAPER="pl_PL.UTF-8"
LC_NAME="pl_PL.UTF-8"
LC_ADDRESS="pl_PL.UTF-8"
LC_TELEPHONE="pl_PL.UTF-8"
LC_MEASUREMENT="pl_PL.UTF-8"
LC_IDENTIFICATION="pl_PL.UTF-8"
LC_ALL=

* launch konsole
* from konsole execute 'kdesu locale':

LANGUAGE=pl_PL:pl:en_GB:en
LC_CTYPE="pl_PL.UTF-8"
LC_NUMERIC="pl_PL.UTF-8"
LC_TIME="pl_PL.UTF-8"
LC_COLLATE="pl_PL.UTF-8"
LC_MONETARY="pl_PL.UTF-8"
LC_MESSAGES=C
LC_PAPER="pl_PL.UTF-8"
LC_NAME="pl_PL.UTF-8"
LC_ADDRESS="pl_PL.UTF-8"
LC_TELEPHONE="pl_PL.UTF-8"
LC_MEASUREMENT="pl_PL.UTF-8"
LC_IDENTIFICATION="pl_PL.UTF-8"
LC_ALL=

(exactly the same as user's)

* launch some kde application, ie. 'kdesu kate'

The application launches with Polish messages, while LC_MESSAGES is set to 'C'.

It looks like LANG variable overrides LC_MESSAGES, which should NOT be the case, according to 'man 7 locale':

----------------------------------------
1.     If there is a non-null environment variable LC_ALL, the value of LC_ALL is used.
2.     If an environment variable with the same name as one of the categories above exists and is non-null, its  value  is  used for that category.
3.     If there is a non-null environment variable LANG, the value of LANG is used.
----------------------------------------

Non-KDE utilities behave as expected, for example 'kdesu tar':

tar:
You must specify one of the `-Acdtrux' options
Try `tar --help' or `tar --usage' for more information.

One of the solutions is actually *copying* settings from user's ~/.kde/share/config/kdeglobals

[Locale]
Country=pl
Language=en_US

...but in absence of settings in /root/.kde LC_MESSAGES should be honored.