I’ve been to both Japan and Korea. They are very different. In Korea you can get by speaking broken English. If I spoke real English, they would not understand, so I spoke it slowly with a fake Asian accent and they understood. Every restaurant you would go to had at least one English speaker. There was a good amount of English signs. Local cuisine was cheap.
In Japan, the only people that knew English were in the airport. And Japan was super expensive. It was really hard to be understood in Japan.
As for getting work there, I can offer no educated guess on that. So I’ll give you and uneducated one. You might do ok as a novelty. Having an teacher from the West is a show of status. But I do know that you will have to get a work visa if you with to work there legally. Many people in Korea are there working on tourist visas, but Japan is much more strict. Before you think of Japan, look into what it takes to get a work visa.
Also keep in mind. Being in a country where nobody understands you and you don’t understand anybody is hard. But I found the hardest part was shopping for products. Pick up a food item, you can’t read the ingredients. Unless the package has a pictures of what it’s is or what it’s for, you are screwed.
I know I answered more than you asked. But they are things to think about.