* bimensuel/elle : twice a month
* bimestriel/elle: every 2 months
And trimestriel and semestriel for every 3 and 6 months respectively.
It's strange that English left this ambiguity in usage when the semi- prefix exists.
We use it all the time here in Australia - eg "This is a fortnightly meeting" / "Lets meet back up in a fortnight and discuss progress".
I consider teaching "fortnight" to americans a gift I insist on giving to every american colleague I work with. Its already in your dictionaries, after all. Please use it, and you're welcome!
It's good for that, but we're in the same position Americans are if we're trying to talk about something happening every two years.
...the ambiguity exists whether or not you use fortnight!
Fortnight combined with semi- will do pretty well, assuming people don't screw up semi-. But you still need an unambiguous term for "every other month" before the problem is solved.
Maybe if you can change the schedule to quarterly?