You can pay with PayPal on the Japanese eShop, and PayPal can convert currencies, so that should work, but now that I look it up, you might need a Japanese PayPal account? I've bought games from the Japanese eShop before, but I don't remember what all I had to do to get it to work.
I can't find anything definite on SmileBASIC 4 outside of Japan, though there's
this tweet from before SmileBASIC 4 was announced.
Personally, my hunch is that it will make it over eventually. Considering they still decided to release SmileBASIC 3 in Europe and Oceania, despite all the trouble they had getting it through the approval process, and the fact that it was really late in 3DS's lifespan by the time it did get approved, I feel like they do care about the overseas community.
I think it'll be easier for them to bring it over this time around too, considering they've got more profitable stuff like Smile Game Builder bringing cash in now. Selling NoE on a programming tool will probably be a lot easier the second time through as well, now that they can point to SmileBASIC 3, Fuze BASIC... hell, even Nintendo Labo. So, there may be a bit of a gap between Japan and elsewhere, but I don't anticipate a huge gap between America and everywhere else this time around.
(Sure, BIG never made it over... but then that was on the Wii U, and
Kobayashi has tweeted that the reason BIG wasn't being brought over was because of the combination of the poor sales of the Wii U and the fact SmileBASIC is niche to begin with.)
