During my Masters in Applied Linguistics I was really challenged in some of my assumptions about this stuff.
The answer is, do not speak anything but the non-local language. If your child tries to speak the local language to you, refuse to give in, literally just feign non-understanding, even if the child knows you speak the local language. Be completely uncompromising.
As to your concerns, don't worry, the evidence is clear, while there is a slight speedbump starting school, kids catch up very quickly. The difference is negligible when compared to the individual variance between different kids in language development.
Accent is a non-issue. Unless you never take your kid out into the local population, their local language accent will be the accent to which they've been exposed to that language. There CAN be some interference in bith directions however. In those cases, you can simply explicitly correct their pronunciation. It will be fine. It will stop being more than a very rare issue once they reach school age.
The bigger question is, what do you do when friends come over? That's something you'll need to negotiate with the child.
Edit: I realise that for a lot of people, this advice seems to be exactly what would be intuitive, however once you're steeped in Applied Linguistics literature, that's not necessarily the case anymore. I post this here to highlught that in this particular context, the intuition most people would have is correct and backed by the literature. Most people's intuitions about language learning are completely wrong-headed I'm sorry to say, but i. This particular case, it's an exception.
Also, question, do you want them to be able to read in the non-local language?
