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Other central banks are other central banks. Their monetary policy does not affect Japan in the same ways the actions of the BoJ do.

Weakening currency is (indirectly) more or less the point. Lack of monetary expansion may very well not be the only reason for their economic stagnation, but it was a hindrance, so this turn is overall very good news; there's certainly a possibility they'll overdo it, but that remains to be seen.