There is now a template for Japanese labels available in the Custom Label Maker. You'll find it here:
On the bookplate creator page, there is a table on the bottom, showing "my templates", "my images", and "my caption". Those words are missing on Japanese page. On "my images" tab, there is no "add" showing although I can see there is a link by placing cursor on. Also, the orange box to follow the instruction is shown blank.
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