I've tried with Thunderbird 52.9.1 it works as expected: the forwarded body has the original body "�にほんご" as-is. So this looks a regression.
Bug 1540943 Comment 2 Edit History
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I've tried with Thunderbird 52.9.1, and it works as expected: the forwarded body has the original body "�にほんご" as-is. So this looks a regression.