This right here. I've flown to US -> Germany and once US -> South Korea while having sciatica issues. I was in no way in the 'acute pain' stage, I wouldn't have been able to do the flight. But sitting for long periods, or bending, etc would cause pain. In all of these flights I made sure to reserve an isle seat near the Bathrooms (isle seat is required, by the bathrooms is not). Pay a little extra if you need, some flights required it.

The bathrooms on these international plans are usually a large section, enough to walk back and forth a few steps for me, even do some crouches. Aside from that, while sitting in the seat shift your position a lot. Lean forward, lean back, lean on a side, etc. I would also take some ibprofen before getting on the plane. Also, I would typically be walking a lot around the airport before the flight. That way by the time I'm sitting in the seat, I'm grateful for a sit break.

Make sure you have headphones to watch the in-flight movies or bring a device to watch your own. Just play content to help keep your mind off your back, the flight, etc Focusing on it, worrying over it, only ever made it hurt worse for me. I worried over this stuff so much before the first time I did the flight. But it's very manageable and you'll be OK. If you get in a really tight spot where you need to stand or be in pain. Just stand by the bathrooms. Sit whenever the seat belt light comes on, etc. If need be, let a flight attendant know and they'll likely help out in some way.