What happens during those events ?

The formal dress consists solely of a suit and a necktie... do we have to wear the jacket too ?

I'd like to attend one but I don't really know anybody who would go with me and what to say and do... what is the etiquette, common practice ?

Can a young bloke like me attend ?

Don't see why not.

If you've been invited, then of course you can go. Just dress smartly and be nice and polite. Which ambassador is it? Just remember they're civil servants not royalty, etiquette would be, don't get drunk, start a fight, throw up etc. etc. unless it's an Australian do, in which case you'll probably not be invited back unless you do!

As far as getting someone to go with you, you should have no problem getting a date for a function like that. If you do get stuck for a date, I'll go with you

Smart, with a jacket will be best.

On the invitation they say:

'dress code : relaxed' (decontracte in French).

Can it be working pants and polo T-shirt then ?

It's the Belgian Embassy, the ambassador is Rudy Veestraeten.