Given Vf across a LED, what is the current draw?
It looks to me that the "3,8" shown on the drawing is the pin numbers of the common anode or common cathode connection, not the recommended voltage.

If the drawing is correct, there are six LEDs in series for each segment.  A typical red LED has a forward voltage of about 1.8 volts, so you will require somewhat more that 10.8 volts to light a segment.  20 mA seems to be a safe current for most LEDs, but they will work (but be dimmer) at lower currents - I'd aim for 10 mA for a start.

If you have a 12 volt supply, you want a series resistor that will drop 1.2 volts (12 - 10.8) at 10 mA, which would be 120 ohms.

For the decimal point, with only two LEDs, the resistor should be abou 820 ohms.

To be certain of the actual voltage and permissible current, you should demand a proper datasheet from the vendor.