How common are SEED certificates outside of Korea? When is support required? I'm looking at the SEED algorithm and would like to know if this is still in common usage.

Can anyone tell me when I would need to implement this standard, either as a client or a server?
I believe the Wikipedia article about the SEED block cipher sums it up nicely.

SEED is pretty much ubiquitous in Korea, and unused outside of the country. I'm not sure if this article is still accurate, but it does give an idea of the history of SEED in Korea: "the cost of monoculture".

If you work with Korean clients, and they demand that you support it, you should certainly implement it. Otherwise, I don't see any reason to do so.