This thread is resolved. Here is a description of the problem and solution.
Problem: Japanese language ISO code is "ja", but you want to use "jp" instead.
Solution: If you want a /jp directory, you need to create a custom language and use the jp code for it. If you need to transfer "ja" content to "jp" language, the best way to do so is bulk duplicating your "ja" content to your new Jp language via Translation Management. After doing so, you can delete your content and even the language, so you don't risk confusion.
Relevant Documentation: https://wpml.org/documentation/support/editing-wpmls-languages-table/ https://wpml.org/documentation/translating-your-contents/displaying-untranslated-content-on-pages-in-secondary-languages/using-content-duplication/#batch-duplicating
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