It's hard enough to understand a codebase, but if a language was like Haskell for instance (you can enable certain extensions in Haskell with a flag at the top of a file), you could have a line at the top of a file defining that this file is 0 based, while maybe the default is 1 based for the language. It doesn't seem like that much more than you already have to consider in many languages.
Funny you put it like that, because the fragmentation regarding language extensions is frequently discussed as one of the biggest issues affecting real world Haskell.
