Sometime you may see different language encoding in X than on your console (tty) prompt. Sometime two different user need two have different language encodings.
~/.dmrc file - Per-user language support
In theory this file should be shared between GDM (Gnome) and KDM (KDE), so users only have to configure things once. This is a standard .ini kind / style configuration file. It has only one section called [Desktop] which has two keys: Session and Language. There are some per user configuration settings that control how GDM behaves. GDM is picky about the file ownership and permissions of the user files it will access, and will ignore files if they are not owned by the user or files that have group/world write permission. Normally GDM will write this file when the user logs in for the first time, and rewrite it if the user chooses to change their default values on a subsequent login.
Setup language encoding in X
Defining LANG variable is not sufficient, you need to setup language encoding using ~/.dmrc file.
[Desktop] Session=gnome Language=cs_CZ.UTF-8
Refer to Gnome Display Manager Reference Manual for more information.