Colloquialisms will pose tricky when they conflict. For example, "3pm next friday" is deciphered by duckling as Friday, 3 October 2014 at 15:00:00 +0000 (UTC) which would not be correct if talking colloquially where I grew up as "next" means not this one but the one after or, more generally, "the one next week not the one this week".
This is probably not the case everywhere, which is why duckling uses next and this interchangeably.
Another reason to know what fortnight means is that it enables you to use the word "fortnightly" instead of "bi-weekly." With the latter I never know if the speaker meant "twice a week" or "every two weeks" without a follow-up question.