Instead of requiring the script file to contain the list of
translations, just extract it from the list of po files on the
filesystem.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>

* when calling from withing git's Documentation dir, using bundler
  might not be desired.
* encapsulating recursive Makefile calls inside bundler breaks the job
  server of GNU make and generates a warning.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>