Use French when no translation
Do you have any data for "the average knowledge of English is quite OK too"?
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  .
What about you ? Do you have any data that shows the opposite ? The Breton wiki has been functioning perfectly well with English as fallback language since 12 years and nobody has ever asked to change that. As far as I know you don't know Breton nor have ever participate to the Breton project but you seem to know better than us what is good for our wiki and our linguistic comunity. One of the great thing about Wikipedia is that it is an international project totally independant from the control of the States and their official languages. It is a free project and I don't know why you want to put our wiki under a French umbrella. Is there a rule on wikipedia that forces every minority language project to use the official language of the State as fallback language ? Is it so difficult to understand that minority language users which are all at least bilingual people do develop wiki projects in order to set up a friendly environment for the use of their specific languages. The very point of this being to escape from the overwhelming omnipresence of the majority language in their everyday life. So, let us be, please.
Yes, I have a lot of data to show that few people know English. A summary is at http://www.unicode.org/cldr/charts/latest/supplemental/territory_language_information.html#FR but we are collecting more, see mw:ULS/FAQ#language-territory for some links (mainly what Seb35 already linked).
I'm sorry you feel this has something to do with states or "a French umbrella"; this only has to do with the language people speak.
I'm not judging whether the Breton wikis are doing well or not so I won't comment on that. I just know that we can do better and my assumption is that if the wiki's interface is in a language that users understand then the wikis are more likely to succeed. Do you disagree with this assumption? If you disagree we can try to test the assumption.
I'm afraid this conversation is going nowhere. I'm not arguing Bretons speak better English than they speak French. Again, this is not my point. Everybody speaks perfectly French in France. But you assume that people want to read fallback French messages in their wiki just because they know French. What I'm trying to tell you is that knowing a language is something, wanting to have it in your wiki is something else. As I wrote before, French is not a neutral language for us, English is. This has to do with traditional French policy of not recognizing any of the languages spoken in France other than French. The answer I give is the same answer you got from the Occitan wikipedian, as quoted by Seb35. Languages are something more than just stats (how many people know this or that). People feelings do count. Anyway, I don't really understand why English should be so great an issue for the Breton wiki all of a sudden ; it works perfectly since 12 years and, in fact, very few untranslated messages remain for everyday users or contributors, it's not as if our wiki was newly created. Besides, and more important, English is the core and original language of producing Mediawiki and Wikipedia messages. Impose an other language on us will cut us from direct access to the original source and oblige us to rely only on translation, which will symbolically and effectively put our comunity as a second level one because it will be definitely deprived of direct access to the original source of information.
So in short, the answer to my question "Do you disagree with this assumption?" is "yes"?
I disagree with your project of changing the actual default language when no proper Breton version is available.
Yes, that was clear. I'm trying to see if there is some principle we can agree on, so that we can star from there to build an agreement. Not answering my questions does not help.
I've been trying to answer your question(s) the best I could for about a week now, with no success I confess, it seems your mind is already made from the start. You want to change our fallback language (because we're French and French people speak French), something nobody is asking for on our wiki (because precisely we're trying desesperately to survive with a litte bit less French around in our life and building an enciclopedy in our language is one of the answers for that). Therefore, I don't see what kind of agreement could possibly be on the table there, or it is English, or it is French.