Are there plans for Japanese-Japanese localization?

This is not a high priority for us at the moment, and we'll likely localize to other languages first (for example, learning Japanese in French, German, etc.).

Some of the members of our team used to be adherents of the "use only Japanese when learning Japanese" approach. After many years of discussing this with one another, consulting with PhD holders in fields related to language acquisition, and interviewing people who have achieved near-native fluency in Japanese, we decided to move away from this approach. Leveraging one's native language when learning a second language seems to be the better strategy.
It must be noted that this only works when getting consistent exposure to Japanese-only materials appropriate for your level, which is why such materials are incorporated into our learning system. Also, translated words and sentences should include notes about differing nuances and uses, which is another thing we do.