This is most probably related to this bug:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=850532

You can work around it by, for example, adding iocharset=iso8859-1 to
/lib/live/boot/9990-misc-helpers.sh, line 234, within the initrd.img. You have
to unpack and repack it, like this:

mkdir initramfs
xz -d -c initrd.img > initrd.cpio
cd initramfs ; cpio -i -d -H newc --no-absolute-filenames < ../initrd.cpio
<modify>
find . -print0 | cpio --null -ov --format=newc | xz -9 --check=crc32 --x86
--lzma2=dict=512KiB > ../new_initrd.img