Version:           1.13.5 (using KDE 4.5.0) 
OS:                Linux

If I press „Replay“, I see this added line:

„On 2010 m. Rugsėjis 5 d., sekmadienis 16:33:28 you wrote:“

But the date „2010 m. Rugsėjis 5 d.“ is incorrect! It may would be the same meaning if you see in English „2010 of September. Day of 5“.
Should be „2010 m. rugsėjo 5 d.“. Because in Lithuanian language the last letters of month changes accordingly the context. „Rugsėjis“ would be correct, if it is used separately.

You may want to try this:
LANG=lt_LT
date -d "2010-09-05 12:34:56"  +%c


Reproducible: Always

Steps to Reproduce:
1. change lokale to Lithuanian.
2. open KMail
3. Replay to Mail

Actual Results:  
Incorrect date format in first line:
„2010 m. Rugsėjis 5 d.“

Expected Results:  
Correct date format:
„2010 m. rugsėjo 5 d.“

OS: Linux (x86_64) release 2.6.34-12-desktop
Compiler: gcc