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As far as I can understand, nearly all Breton speakers also know French. When there is no translation in br, I assume users would prefer to see a translation into French rather than an English string. Can we configure MediaWiki to fallback to French?
Based on the geographical area where Breton is spoken, yes, almost all Breton speakers speak French but possibly not English. So French is probably a better fallback than English.
I strongly oppose that view. The very purpose of this wiki is for Breton speakers to live in a no-French environment, in a free project disconnected from State or official languages. That is, this is a non-French language wiki and we assume that. We don't want French on our wiki, we have enough French everywhere in our daily live. There is no problem of English being the fall-back language. I'm not aware any member of the Breton wiki has ever asked to change the fall-back language since the set up of the wiki.
Actually, the official purpose of MediaWiki locales (and most localised wikis out there) is to make wikis accessible to the maximum number of users possible. Can you explain how showing more English strings helps reach more people (and Breton speakers in particular), or vice versa how showing French makes it impossible/very hard for some users to use the wikis?
You are missing the point. Almost everybody understands French on the Breton wiki except for some foreigners who learned the language but the average knowledge of English is quite OK too, at least to be able to read non translated wiki strings (everyone in Brittany does at least 7 years English at school). The point is which language do the Breton users prefer to get as fallback language. For the moment it has always be English and nobody has ever complained. So why force French on us ? French is not a neutral language on Breton wiki.
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.
As you know Breton wikipedia is a small one, based on few people so it's not easy to translate everything, even if the result is not that bad for br. What I'm afraid of is that it will result a wikipedia half br/half fr because people will understand French sentences and won't tell us anymore there's something important to translate. So please, let to fallback Breton to English, since the beginning there's no problem with that.
Intentionally annoying people when not necessary in the hope that they do something good is not a tactic that we encourage in MediaWiki.
Fulup and I have been translating a main part of what you can find in Breton here. We know Breton wiki for years, and we know how it works as we are part of it, so could you please listen to our answers to your proposition ?
I'm grateful for your translations, hence I'm listening and I'm seriously considering all your arguments. I heard contradictory arguments about what annoys people more, an English or a French string, and whether such annoyance brings people to translate more; my answer is simply that, whoever is right, this argument is unacceptable.
"What worked so far" is also not an acceptable argument, because it's based on a biased sample (those who could live with the [almost certainly] sub-optimal configuration this language had, differently from all the others).
Back to concrete proposals: if you need ways to find out that some new messages urgently need to be translated, that's a very important issue and we need to work on a feature in Translate that fixes it systematically. The current system only provides two ways: using Special:LanguageStats to translate certain important groups completely; and visiting Special:Translate/!additions regularly to see new messages. You're saying you'd need something else or something smarter, correct? What kind of system would you like to have and how often would you like to check (or be notified)?
Thank you for listening and considering all our arguments. I have a question for you. How is it that French citizens, knowing perfectly French and having an excellent French language wiki at their disposal choose for some of them to give thousand of hours of their time to translate strings or write articles for a tool they perfectly know could never ever compete with the French one. And why is it that some others prefer to read best bad Breton articles than consulting directly a good one writen in French? This is one thing. Secondly, you talk about improving the way of finding messages to translate. Seems a good idea to me although I do appreciate already Translatewiki since years because I was already here working for Breton when it was called Nukawiki. So I value all the impovement that have been made since the beginnig. The thing is it is still very uneasy to find a particular string because one can't make a globabl search. You have to know in what section the string is. So one has to search in wiki strings, and then in wiki extensions, and so on. Although, it is not possible to search from a part of a word only, which is very bad in particular for Celtic languages where the first consonnant of a word change depending on grammar. For example if I want to search the word kemennadenn, I have to search it three times, I need to get kemennadenn, gemennadenn, c'hemennadenn and this in every possible section. This is time consuming. And of course, the big news is that now we have French language in our wiki where it used to be English. So now it is just impossible for translators like me to find the string we want to translate because we see a French string on our wiki and we don't have a clue of what is it we have to search in English because we don't have access to the original string anymore. May be you can understand, this is not what I call an improvement for the Breton wiki. And last but not least it would be very nice, in order to homogenize terminology more efficiently if it were possible to replace one specific word in a more systematic way, a bit like the search & replace function in Word.