Theres no easy answer to this question nowadays, with so much "american english" slipping into the web sites and global shipping of parts.
Axminster tools, one of the largest tool suppliers in the UK, advertise 1/2" router bits, where they should be advertising the 12mm version (or stating the variance).
i have a Makita 1801 purchased in the UK, but have several "1/2 inch" bits as well as others purchased here in Cyprus, which is a fully metric country. But of course many tools here are imported from the USA.
Its a completely circular question and answer.
The ONLY way to be safe is to check the cutter bit in the collet. If its a smooth sliding fit, youre good to go. If its flops about, dont use it. And that applies to any size router bit any time.
Just to confuse matters further, i have a selection of adapter sleeves and in my "1/2 inch" router i use bits of shank sizes 1/4", 3/8", 1/2" as well as 6mm and 8 mm.