Lithuanian diacritics were mapped as follows:
ą → AltGr + a
ę → AltGr + e
ė → AltGr + r
ų → AltGr + u
ū → AltGr + y
ž → AltGr + z
š → AltGr + s
# clone the repository
git clone https://github.com/tmneth/xkb-lt.git
# patch xkb-data
./patch.sh
You can always restore the original xkb config by reinstalling xkb-data
or xkeyboard-config
depending on your distro.