Cannot access filenames containing non-ASCII characters.
Steam Deck's SteamOS 3 provides only one locale named en_US.utf8 (as configured in /etc/locale.gen) but /etc/locale.conf sets LANG to en_US.UTF-8. They don't match, so the effective locale for programs is C and the character set becomes limited to ASCII.
Set the game's launch options in Steam to LANG=en_US.utf8 %command% to solve this.
To fix this for all applications running in desktop mode (eg. to correctly extract games), create a file named ~/.config/plasma-workspace/env/utf8.sh containing (restart desktop mode to apply):
