[jhowell]

It is odd that the translation dialog comes up, but not the dictionary.

Is Japanese shown as the current language in the "From" section of the translation dialog when that happens?

Added: After further thought, it seems likely that the absence of the dictionary dialog indicates that the Kindle has incorrectly determined that multiple words have been highlighted. There may be issues with that in languages that do not use spaces between words. Japanese and Chinese books published on Amazon include a table of word boundaries that will not be present in homemade sideloaded books.