My own experience being here in Switzerland is that 8mm definitely IS common. Some time back I found out that if you take any router (choose any make you like, it need not be black & green!) if you buy it in UK (and probably in the US too, dunno for sure) it will come with Imperial collet/s as standard. Buy exactly the same make/model here "on the mainland" and it will be Metric collet/s as standard.
Remembering that collets are designed and made to very close tolerances, it will NOT work to try, say, a 12 mm router in a half inch collet, or vice versa. Same goes for all other router sizes and collet Imp/Met sizes. And the problem is not helped by some sellers (e.g. Amazon) sometimes not stating the router shaft size in their sales/spec write ups. People like Wealden, etc, are MUCH better for that.
So as someone has already said above, it makes good sense to hold a stock of collets in all of both the Imp and Met sizes you think you'll ever need, especially if like some of us, you're likely to have "imported" an Imp Router (or bits) from UK to Europe or vice-versa.
Collets are certainly NOT cheap, but if you're likely to be in any of the above situations, they're a lot cheaper than a hospital visit/amputation because you've mounted a Met router bit into an Imp collet (or vice versa) and didn't realise it hadn't tightened up fully.