A short post in the hope of saving other people time – UE4 materials currently (in 4.7.5) do not support reflection/specular and translucent on the same material at the same time. Turning on translucent disables reflection and specular. So you’ll have to fake the reflection somehow if you need them, possibly using some clever texture manipulation.
EDIT: This tutorial has the clever texture manipulation – https://wiki.unrealengine.com/Glass_Material_-_Video
I haven’t tried it but from the video it looks good!