Whenever packaging project from Launcher with a PreviewGameLanguage set, the culture="preview language" is added to the UE4CommandLine.txt file.
Can avoid this issue by setting "None" in PreviewGameLanguage. However, this seems like bug as the user is presented with an unintended language. It is better that the PreviewLanguage does not affect the applicaton package.
Always launch culture "ja".
Launch in "en" (Default language or OS language)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-103006 in the post.