A country with any level of unemployment, already has a bigger workforce pool than it knows what to do with.

I am not saying that xenophobia isn't bad, but it's a bit of a leap to say that Japan is in any danger of a shortage of workers.

You can argue that countries should aim for full employment, so any unemployment by definition means they're mishandling things. But certainly increased labour supply doesn't automatically increase unemployment rate in market economies - it often has the opposite effect. Otherwise, smaller countries would tend to have low unemployment and populous countries would have high unemployment!

Why do you think Japan's population is in a death spiral?

Birth rates are falling across the board in the first world, but unlike the west, they have few immigrants to make up for it.