The bug made it so you couldn't select or create anymore icons in any series I think, since the Mother 3 icons were exclusive to Japan's version of the NSO app. They added them to the USA version (and maybe others), too, though, so I guess that fixed the bug. I'm not sure why it kept you from other series' icons, but it's all good now. I assume since they were Japan-exclusive, and they weren't present in other versions of the app at first (but were still in your icon inventory), they broke the icon system or something like that. I didn't experience this myself, however, since I waited until I read it was fixed to even try.
If you mean how it works to let you get them, you just need to change your Nintendo account's region through an internet browser on the My Nintendo site. Go to your account's settings, then edit your user information.
After that, the NSO app on your Switch should automatically update, but it'll require you to log in again, and it may need to restart the app before that. Everything should be in the same spot it's in within the English version of the app, just it's all in Japanese now