Proper old-fashioned spooky stuff and a creepy cliff-hanger. We'll see what happens in the second half...
It was good to see a Deaf character on Doctor Who done properly, even if their sign-space wasn't always in frame (which is what always happens with hearing directors). The interpreter was less convincing, for all the usual reasons (ability to see through solid objects, translate things before they've been said, and so on), but that was easier to ignore. Also good to see lipreading portrayed as unreliable in a plot-critical context.
We came to the conclusion that the TARDIS doesn't translate languages across modes (it'll canoncially translate writing to writing, and speech to speech, but presumably since neither Clara nor The Doctor know a sign language, it doesn't intervene). I assume it can translate all programming languages to Fortran, if required