How do I change my Xbox language?

You can change your system's region/locale within the settings

1. Sign in to your Xbox console.
2. Press the Xbox button to open the guide.
3. Select Profile & system > Settings > System > Language & location.
4. Select Japan from the list, and then select Restart now.

This will change your entire console to Japanese, and when games read the locale, it will change the game(apex in this question), to that language as well. It will not affect connection, as that is still primarily ping based.

This method is not recommended if you can't read Japanese, but it is the only way for the console to achieve your desired effect of Japanese voices.

Some games offer language options in the game itself, the best hope is that Respawn eventually does the same.