Like in switzerland they speak french, the code is fr-ch.
Now what happens when a swiss person visits the site?
Do they get the default locale (1) or does it switch over to fr-fr (2) ?

2 seems to be the better option as they could understand it, instead of seeing a blog in a language they don't understand.

So yeh, if the specific language doesn't exist does it default to the standard form of the language, or to the blogs default language?

Cheers.

Edit:
Gave it a whirl, and found out that it keeps going through the users locales until it finds one it can use.

Maybe a good idea is to show a message saying that the blog has defaulted to the users 2nd or 3rd or whatever locale instead of their first, and maybe they should see if they have their locales set properly...