@Remi The NA version is a newer version, its based on Episode 6's client and is only 'gated' to appear like as if it were the older versions, albeit with lots of Episode 6's features. In comparison, while the SEA version was still on Episode 2 (around "version 2"), JP was on Episode 4 (around "version 4"). It was already in English yea and indeed large majority of the well translated parts were taken from Sega of America's original cancelled 2013 version... but the rest is a horrible translated mess and lots of 'find and replace' was applied to make series terminology more generic (Monomate = Health Drink, Force = Wizard, etc). Also since we were discussing the launcher... it did still use the same launcher as JP albeit modified.
Keep in mind also... whether they used that version or not has little to do with the issues going on now... simply using an older version does not automatically fix the issues relating to Sega's implementation of MS code calls, App Manifest issues, etc... thats all Sega's improper setup of the game installer on the MS Store.
As a very quick comparison... here is the JP launcher for Episode 6:
and here is the NA launcher:
For comparison with SEA here is the old JP launcher:
And here is the SEA launcher: