I did what you said but nothing better. There is still a problem with the dropdown langages selection list in admin when I want to add a translation (../content/translations).
The list is like that :
Custom
------------(a blank item in the list in first position)
Catalan
Chinese ...
The blank item should be "Breton", the new custom langage ?
Check language name in ezcontent_language table in database and make sure that SiteLanguageList for your current admin siteaccess contains correct locale with new language. It should work.
Check also what is in: $GLOBALS['eZContentLanguagePrioritizedLanguages']