It doesn't mean you have to come back to St Jean all over again. It means you have to have your profile re-evaluated before the expiration of your 5 year limit. Then your profile is then validated for another 5 year period from that date.
Make it your business not to lose your second language capabilities in this 5 years (or ever) and you'll do just fine. Want to make Snr Officer, then make sure you use that French that the taxpayer has dished out the dough for you to learn, even while you are posted to an English speaking Unit. There are plenty of Franco or French speaking personnel at every base. If you've learnt it; use it. Then you've got no worries and no one to blame but yourself if you do lose it.