Bug 19522: Label creator: edit batch: make "Export label" translatable

Bug 19522: Label creator: edit batch: make "Export label" translatable

Test plan:
1. install the fr_FR locale
2. go to /cgi-bin/koha/labels/label-manage.pl?label_element=batch
3. create a batch if there is none
4. edit that batch
5. Switch the language to French if you haven't done it yet
6. click on "Exporter le lot entier"
7. then you should see "Export labels"
8. apply this patch
9. install the language
10. replay steps 2 to 6
11. then you should see "Exporter des étiquettes"

Signed-off-by: Simon Pouchol <simon.pouchol@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>