Here a couple of screenshots. I've circled Breton in red and Welsh in yellow.
From what I've read (I am not a very experienced editor), I think you might be right about it being fallback languages. Welsh and Breton are not good fallbacks for Cornish speakers as the languages aren't similar enough to Cornish for Cornish speakers to understand. (I speak Welsh, so I can cope with that, but I don't think all that many can! I don't speak Breton and so I have very little idea what on earth is going on when I see it. To give you an idea, the only words I can work out in the second picture are "pajenn", "gwelet" and "ezhomm", which means I'm pretty clueless!)
On the other hand, I have not yet met a Cornish speaker (and I know many many Cornish speakers) who are not fluent in English, so I'd suggest using that as a fallback language.
I am definitely up for doing some translating into Cornish here to help Cornish get its interface in its own language! I currently mostly use Cornish Wikipedia to use the translation feature to translate articles into Cornish, so for me that'd be a good place to start. But I'm new to translating the interface itself, so I wonder if you could point me to the right section to get started doing that? Thank you so much for your help!