In order to ?debug translators and being able to step into glibc during it, on Debian you need the hurd-dbg and libc0.3-dbg packages installed. If you need to debug the initialization of the translator, start the translator like

$ settrans -Pa /foo /usr/bin/env LD_LIBRARY_PATH=/usr/lib/debug /hurd/foofs

The -P option will make it pause and you will be able to attach ?GDB to the process.