For DS games, especially DSi Enhanced, they are split into these regions:
- J - Japan
- E - America
- P - Europe
- K - Korea
- O - Undocumented
There are more regions but I see no reason why these are relevant here.
The English versions of Black and White, as well as European versions, all have these ID codes on the box:
- TWL-P-IRAO-USA, EUR or AUS
- TWL-P-IRBO-USA, EUR or AUS
Had Game Freak region locked these games, then these would be IDs:
- TWL-P-IRAE-USA (or IREP-EUR or AUS)
- TWL-P-IRBE-USA (or IRBP-EUR-AUS)
DSi and 3DS consoles will search for whatever region is inside the cartridge. "O" is an undocumented region which is recognised on all DSi and 3DS consoles, so that means any copy of Black & White and Black & White 2 with this will be fully playable on any region DSi and 3DS console. Adding to that, when the region code is O, then the actual region inside the rom will normally be blank (Black & White International are the only games to my knowledge that has this region code).
So if you take a BW ROM and open it up (forgot what program), instead of seeing this:
TWL-IRAE-USA or
TWL-IRAO-USA
You will get this:
TWL-IRAO-??? (or blank, so there is no region assigned because O is not a documented region).
I can confirm this as I have NA copies of Black & White and they are fully functional on my DSi and 3DS (European / UK). Adding to that, when the ROM of English version of Black & White was dumped, it was discovered that the European and American versions of these games were bit-for-bit identical with the exception of one dump lacking the DSi enhanced portions. Had both the EUR and USA games been dumped, then I am fairly sure they would have been fully identical but that is irrelevant at this point.
I've seen a screen-shot of the White 2 cartridge, and like BW before it, the ID is TWL-IRDO, so there is a 90% chance that English copies will in fact work on any region DSi and 3DS console. I have also asked a blogger who also has the game and they have confirmed the ID I have provided them.