It's a bad idea to set the current culture in the convention.
If it's get used in code other than Fixie itself it can cause false positives in tests of the users.

It would be better when the expectations would also be using the current culture or ignore the format of the duration.
Another option would be to fix the output methods like you did for the reporters.
I would prefer the later one, currently all messages are not localized but parts of it like numbers are, we should not localize them also.