Modern systems have a well defined global default keyboard layout, handled by the localed service (part of systemd). This is the layout used by display managers when logging in locally. ThinLinc however has its own default in sessionstart.hconf.
Having the correct layout configured in ThinLinc is not critical, but it does avoid some odd behaviour. So it might be worthwhile for us to respect it system default (at least until the admin configures something different). It is more likely to be correct than the "us" we currently ship.