From what I recall from schematics I've seen, the ST-Link does have level shifting for the incoming signal, but none for the outgoing signal. Which is fine, as the SWD and JTAG pins on STM32 are FT, so when your STM32Lxxx is running at 1.8 Volts, it accepts input at 3,3 volts.
When I look at the I/O levels for the STM32F103:
V_IH for standard I/O pins : 0.41*(V_DD -2 V)+1.3 V
V_IH for FT I/O pins: 0.42*(V_DD -2 V)+1 V
When running at 3.3 volts, this gives 1.833 V for normal I/O pins, and 1.546 V for FT I/O pins. So, looking at these numbers, it looks like it should work even without level translations.