BID PRN means you can take it a maximum of twice a day when you need it (preferably as close to 12 hours apart as possible).
It does not mean you can take it twice a day OR however many times per day you feel you need it.
If you have any questions about prescriptions, sometimes pharmacists are a better resource than the doctors. It's their job to know the drugs and stay current on them.
0.5 mg is a very small dose, but it might take the edge off a bit to allow you to sleep.
Be careful about taking benzodiazepines like this, you can become addicted if taken routinely for a couple weeks. And withdrawal can be a bitch.
Good on you for being interested in the meds you've been prescribed. It's important for people to research things like this to look out for their own safety. Mistakes can be made even with numerous safe-checks in place in the healthcare system.
Here are some pages where you can find out more about Ativan (lorazepam)
webmd medicinenet.com
medline
Question: Are there exclusively English-speaking doctors in Montreal? Are pharmacists expected to be completely bilingual so they can read scripts in both languages?