[Mediawiki-i18n] Good and bad fallbacks

[Platonides]

What would be the right way to differenciate between fallbacks like
Picard -> French (there's a similarity between the languages, the
message might even be the same) and French -> English (which is there
just for having a base language). Take the fallback lists like shown in
the message file, I guess? What's the prefered way to do that? Shouldn't
we add in that case another parameter to Language::getFallbacksFor() to
disable the addition of en as final fallback?

Regards