OK, here's what I tried:
I uninstalled Fabrik completely from the Joomla system. I then downloaded a fresh copy Fabrik 3.1b from the website, and installed it into Joomla. That went well, and the labels were there, as expected.
Next, I downloaded a
fresh copy of the Github files, extracted them, and uploaded them. I went to Extension Manager > Manage, selected all the Fabrik files, and performed a "refresh cache." I also went to the Discover screen and installed all of the not-yet-installed parts of Fabrik.
I then went to Components > fabrik > forms and tried to create a new form. The following screenshot shows what I'm (still) getting:
I looked at the file you mentioned (/administrator/components/com_fabrik/language/en-GB/en-GB.com_fabrik.ini) and do see those elements there. But for some reason, they're not being picked up.
I also see that the text used for the mouseover on the button labels is in the file, and it changed when I changed it in the file.
It seems as though where's something not right in the file which generates the form buttons page in the administrator area for the extension.