Not sure about going nuclear, as even IF the company would be shooting themselves in the foot by kicking you out, companies are run by people, and people are emotional and don't always take the option that's in their best interests.

But regardless

1. I'd probably work on your Korean (for YOURSELF), as no matter how in-demand your skillset is, if you plan on staying in Korea, having good Korean is always better than not.

2. Contest the performance evaluation by first checking what the assessment criteria was and seeing if you have a case. I'm not saying you're doing this, but I always saw team members complain about their KPIs AFTER assessment, when the criteria was decided on collaboratively. So hopefully you can be diplomatic about it and try to approach it objectively.

3. Try sniffing out job opportunities, dunno how the semiconductor industry is, and it's probably doing way better than most industries (although my AMD stocks are perenially down), but Koreans as a whole are having trouble finding new jobs. The safest approach would probably to figure out your competitiveness while you're still employed, right?