The Feral Launcher requires libidn version 1.33 in order to work outside Steam Runtime mode, check if the Linux distribution has it in their repository, normally it would include the version number or a compat highlight, sometimes even both (Such as libidn-133-compat in ArchLinux's/ArchLinux based distro's AUR). If it is not present, it is possible to compile one from GNU's page, which also includes the instruction manual and commands that can be copied & pasted. Bear in mind that you should use sudo command when you wish to install the compiled library.
A much safer option is to use the LD_PRELOAD environment variable in Launch options if you fear that it may replace the recent version of libidn, after you have compiled but not installed it.
Alternatively you can create a symlink of libidn.so.12 as libidn.so.11 using
ln -s /usr/lib/libidn.so.12 /usr/lib/libidn.so.11
as superuser/root if the package is not present.