Recently I ran a rototiller over my water supply line, a 15mm copper pipe with disastrous results.
All I have to patch the break with is US 1/2 inch copper and fittings.
Am I going to have problems connecting US to metric??
Not sure if you will have success. I have not worked with metric fittings but 1/2" is the same as 13mm and 15mm is about 9/16 but not exactly. However my bigger concern would be that your water service is in the ground at a height that would allow it to be hit by a rototiller. Here in the North East we must be about 4' down to prevent freezing but even if freezing isn't an issue it should be low enough where its not going to be easily damaged by digging or rototilling. What part of the country are you in? I would seriously consider digging the trench deeper if possible so you or a future owner do not have further trouble. If you can't you may want to try a plumbing supply house as they may be able to get you the correct size parts as opposed to one of the big box stores. Hope this helps you out.