[Xfce-i18n] How can I add a Dutch translation file for tumbler?

[Pjotr]

I want to add a Dutch translation file for the tumbler package. This
is in master, but needed bij xfce-4.8. In the current Dutch
translations list it doesn't show either in master or in xfce-4.8.

I tried adding a file called tumbler.master.nl.po on this page, but to
no avail (no error message either):
https://translations.xfce.org/projects/p/tumbler/c/master/

How can I add it, so that I can start translating by means of the web
interface of Transifex?