Use French when no translation
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.