No, Ukraine did. South Korea just backed them up.

See https://lemmygrad.ml/post/6007978:

It started when Kyiv Independent quoted a “Western source” that the DPRK has sent 10.000 troops to Russia

...

The latest “evidence” posted of these supposed DPRK soldiers “fighting for” Russia is two videos without a date or location, but purportedly from Russia’s far east. In both videos you can hear Korean being spoken. In one video they are outside training, and in the other they are receiving Russian uniforms.

The Western media can’t even get their story straight though. First they reported that South Korea’s intelligence has said 12000 DPRK soldiers are fighting for Russia in Ukraine but then they revised their story to say 1500 DPRK troops are fighting for Russia. If you look at the CNN url, you will see that it still says “12000 troops”, although the title and the body of the article have changed.