Using a camera on public property in the EU is broadly very legal.
Less and less so; at least here in France and in Germany and also in the UK, which was quite surprising to me. In the EU, the GDRP being another nail in the coffin of the right of photographing on public space and photographing random people in that public space. Most of the cases I've heard of in the last few years ended up with the plaintiff winning against the photographer, even if the picture was not exploited professionally.
Smart glasses will raise a new flag and push all rules to the next level of paranoia (rightfully so, I'm afraid) and will then be used as an excuse to remove even more of our liberty to use public space (which is supposed to be ours).
Edit: clarifications.