Sounds like some good investigation.

To ensure that the issue appears "corrected", the new languages_id for Japanese though now is 3 correct?

In the additional testing, was a new export file created after reinstalling the language pack? And do the fields of the japanese language have the current/new languages_id?

Before, English was 1 and japanese was 2. Because japanese was removed and then readded, I would expect japanese to now be 3.

If the problem continues with a new export/import, I will take a look at duplicating the issue even though I can't read Japanese. I took some lessons many years ago, but that was conversational not "business".