The conference will be held at the Orea Hotel, while the workshops will be held at Charles University.
ETAPS has a negotiated room block at the Orea Hotel at reasonable prices (95 and 105 Euros a night for a single- and double-bed room, respectively), and there are quite a few other properties nearby. Prague also has very rich AirBnB offerings at all price levels (booked several months in advance apartments are in 50 Euros range near the conference hotel).
Praha 6 169 01
Czech Republic Visit website