did you look at /etc/X11/xorg.conf.d/40-* ?
do you have xorg-xf86-input-evdev installed?

Thank you ernia for your help. xorg-xf86-input-evdev was installed. I looked at /etc/X11/xorg.conf.d/40-Keyboard.conf and changed XkbLayout to pl. There are now polish letters in leafpad, but still not in xterm. It's better then setxkbmap command anyway. Changing XkbLayout to pl2 freezes the system.

you talk about "polish letters in leafpad, but still not in xterm".
could not it be an issue with xterm fonts?
they are set in ~/.Xdefaults, you could verify if something change using the same fonts that leafpad uses.

Thanks for the idea with fonts. Changed to Monospace Regular, but it does not helped. The default DejaVu Sans Mono has all polish letters in stable. Still missing fonts in xterm.

Thanks ernia your tips helped me to discover the problem. Xorg has changed recently it uses evdev to detect hardware. In 3.0 evdev was not installed by default. Now uninstalling evdev freezes system. On arch wiki https://wiki.archlinux.org/index.php/Xorg is a very good documentation on evev with examples for changing keyboard layout. In short you have to comment out the definitions of keyboard in xorg.conf and add one in section "InputClass" of 40-evdev.conf

Section "InputClass"
        Identifier "evdev keyboard catchall"
        MatchIsKeyboard "on"
        MatchDevicePath "/dev/input/event*"
        Driver "evdev"
        
        # Keyboard layouts
        Option "XkbLayout" "pl"
        Option "XkbVariant" "qwerty"
        Option "XkbOptions" "terminate:ctrl_alt_bksp"
EndSection

But qwerty variant of pl keyboard is unavailable on SliTaz, changing to this variant freezes the system. Without XkbVarriant are no polish letters on terminal. Seems that polish keyboard will be unavailable on terminal on next version of SliTaz. This makes impossible operations on files with polish names on terminal. Because I really like SliTaz, would like to change this, but have no more ideas. Dumping a keymap from debian gives nothing. Changing hal configuration makes no sense from xorg 1.8, as it uses evdev to detect hardware. What else can I try?

i would start from here: /usr/share/X11/xkb/*
i can't help you more than this, it's a mistery to me, i casually encountered this dir while fiddling with an egalax touchscreen ...

Thanks ernia for taking your time to help me.
I will try to learn more about keymaps and report about any progress. At the moment all current keyboard key definitions for Polish diacritic characters generatet with altGr and corresponding non-accented letter read from xmodmap -pke | grep acute seems to be diffrent from Debian. Maybe that this double description has no meaning.
xmodmap -pke | grep acute
Debian

keycode  32 = o O oacute Oacute oacute Oacute
keycode  39 = s S sacute Sacute sacute Sacute
keycode  47 = semicolon colon dead_acute dead_doubleacute dead_acute dead_doubleacute
keycode  53 = x X zacute Zacute zacute Zacute
keycode  54 = c C cacute Cacute cacute Cacute
keycode  57 = n N nacute Nacute nacute Nacute

Slitaz

keycode  32 = o O oacute Oacute 
keycode  39 = s S sacute Sacute
keycode  47 = semicolon colon dead_acute dead_doubleacute dead_acute dead_doubleacute
keycode  53 = x X zacute Zacute 
keycode  54 = c C cacute Cacute
keycode  57 = n N nacute Nacute

Anyway i have found a potentially source of this problem. Xterm don't runs in utf-8 mode
I run xterm -u8 got an error

locale not supported by Xlib, locale set to C

but Polish letters appeared on xterm. Have to change system setting now to make xterm start in utf-8, but don't know how.
Hope that someone else can chime in, cause for me it may be a long way to solve this problem.

Error message explains everything polish locale is not supported by Xlib, xterm runs in C locales charmap, that is ISO8859-1. The system locales charmap is correctly set to utf

$ locale charmap
UTF-8

Somebody knows more about Xlib, and how to change it to enable support for polish locale?

There are two simple solutions, that make possible Unicode support in xterm, without typing xterm -u8 in console.
1. Changing desktop file to uxterm
# nano /usr/share/applications/xterm.desktop

[Desktop Entry]
Encoding=UTF-8
Name=XTerm Terminal
Name[fr]=Terminal Xterm
Exec=uxterm -ls
Icon=xterm.png
Type=Application
Categories=Utility;Terminal;

This setting will work on next login.
or method
2. Changing xterm encoding to utf-8
$ nano .Xdefaults
add a line at he end of XTerm settings

! unicode initially on
xterm*utf8:             1

Surely this don’t solves the problem, but makes things a little better.