Use French when no translation
I’m not a Breton speaker so my opinion is less valuable than others’, but I live in Bretagne and I’m a bit aware of the Breton culture and possibly I will learn Breton some day (and I even contributed a bit to the Breton Wikipedia under the pressure of Breton-speaking friends :). I know the France had an aggressive policy in the previous century to reduce the influence of the Breton and other regional languages (as stated on this other thread) (and if you want to know something from my personal history, my grandfather was the last in the family to speak some Breton and I never heard him speaking, probably because of this aggressive policy during his childhood).
However, if we see this question with a pragmatic point of view, the vast majority of Breton speakers are either French native speakers or live in a French environment, so they are likely to understand the French, and on the other hand, and although the situation is improving particularly for young people, a lot of people in France and in Brittany speak badly or no English (*).
So I would be hardly convinced that French would be a worst fallback than English from a pragmatic point of view – in the sense there are more Breton speakers understanding French than Breton speakers understanding English – so more people would be more enclined to use or even edit the Breton Wikipedia if they can understand what is displayed on the interface. I do not know if some Breton speakers would stop using or editing the Breton Wikipedia if they see some sentences of the interface in French.
But as I said, I’m not a Breton speaker, so I let the Breton speakers decide. And whatever will be the result of this discussion, improving the number of translated messages would be always better than relying on a fallback language. Breton is already quite good, I see 50% out of 30 000 messages for MediaWiki (well done!), the main part which could be improved is the extensions used on Wikimedia (42% translated out of 13 000 messages, remaining 6 000) – other extensions are less important from a Wikipedia point of view.
(*) I have no hard data, just feeling and I always heard the English level of French people is very worse than in other European countries, but when searching an answer I found interesting reports about multilinguism in Europe  .