Regarding the Japanese language pack:

Is that for the catalog side, the admin, or both?

After installing the Japanese language pack was the default language changed/modified? Or perhaps better asked is what language settings were modified?

Is the default English language still present? Is it considered language_id of 1? (should be able to see this information on the main screen of tools->Easy Populate v4).

There is a slightly more up-to-date version at https://github.com/mc12345678/EasyPopulate-4.0) where there were some changes to some of the language processing; however, please answer the above questions to ensure successful support. There may still be something needing modification.

The error message is indicating that at some point in trying to match the record with a database entry that a variable substitution is not being made for the "phrase" ':products_model:'.