Data submitted via dialogs in Japanese (Shift_JIS) systems is corrupted

1) Set up JIRA from scratch in Japanese. The default encoding is Shift_JIS.
2) Create a project and an issue.
3) Add a comment to the issue using the comment dialog　(all dialogs are affected, this is the simplest repro step). Write your comment in Japanese.

4) Submit the comment.
5) Look at the characters in the new comment on the view issue page. They are different.

There appears to be an encoding mixup here. All dialogs are in UTF-8 and the View Issue page is Shift_JIS, but the data isn't being correctly converted from one to the other.

When the dialog is submitted, it POSTs the data as URL-encoded UTF-8:

When the View Issue page is generated, it drops those UTF-8 bytes into a Shift_JIS page without converting to Shift_JIS:

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By contrast, it works fine if you submit the same comment using the bottom comment box, which is non-dialog.

becomes

In this case, the POST is done as URL-encoded Shift-JIS:

Which then fits nicely into the Shift_JIS-encoded View Issue Page: