It appears that autogen.sh doesn't like es.po if it isn't mentioned in

[tinc] / doc / sample-config / tinc-down

#!/bin/sh
# This file closes down the tap device.
# Note that if you use the universal tun/tap driver, you don't
# need to do anything; once tinc quits the tap device is already
# removed by the kernel.

ifconfig tap1 down