Germany - wire 3 core garage to 4 core mains
It's hard to say.  Euro power is 3 phases of 230V each arranged in 3-phase "wye".  (whereas America said "Wye not!")  Phase-center is 230V. Phase-phase is 400V. Draw an equal triangle with 230mm from the center to each corner. Now measure corner-corner. You got it. 

Germany is fond of delivering all 3 phases to houses.  So one possibility is that someone brought 2 of the 3 phases to the garage for 400V tools.  In that case simply terminate it off.

Another possibility (common American trick) is that the second phase wire is switched.  Say there are lights in the garage or outdoor lights, the second phase is meant to be used to feed them.  Then they can be turned off from the house.  If all your garage outlets/lights are currently under the control of the switch, then check the black wire - it may be always-hot.  Move the receptacles to that wire instead of the switch.  Yes, it's OK to share neutral in that case.