I would wait until I had a year under my belt before asking anything, but that wouldn't leave much time.
When you do discuss it, just be upfront about it. I'm moving to XXX country around YYY month, is there any possibility I could continue working for the company in the branch there?
But don't count on this job, it's a big ask and depending on the country web developers are probably thick on the ground already. It also depends what sort of autonomy the branch there has in hiring, many companies don't get involved in the hiring/firing of overseas branches. Still worth asking though.
Check pay as well, depending where you are going to there may be a huge disparity from what you are making now. I know people doing comparable jobs in a few countries, but locals here make about 6 times less money than their counterpart in New Zealand or Australia. And for that reason alone a branch here would not want an outsider foisted on them.