First, check in .config/rabbitvcs/settings.conf to confirm what is your language. Mine was:
[general]
language = en_US
enable_attributes = True
enable_emblems = True
enable_recursive = True
show_debug = False
(...)
Then check under /usr/share/locale/en_US/LC_MESSAGES/ to confirm there is no RabbitVCS file. In my case, I just copied the en_GB version:
sudo cp /usr/share/locale/en_GB/LC_MESSAGES/RabbitVCS.mo /usr/share/locale/en_US/LC_MESSAGES/RabbitVCS.mo
Then restart nautilus with:
nautilus -q
Hope this helps.
update for Ubuntu 20.04: I've recently updated my Ubuntu and this fix stopped working. As a workaround, I've just removed the RabbitVCS.mo corresponding to my language in:
/usr/share/locale/pt_BR/LC_MESSAGES/RabbitVCS.mo