Since this game is already out on the Japanese eShop, and Breath of the Wild can switch between Japanese and English by changing system region, does anyone know if Puyo Puyo Tetris has the same functionality? Is there any signage on the eShop to show which languages are supported?

Puyo Puyo Tetris does not have English language support. On a game's page in the eShop, you want to look for 対応言語 (Supported Languages). If it lists 英語 (English), then it has English language support.

@razzuel: Cool, thanks. The lack of English language is no huge deal for me, but I would like to support the localization. Here's to hoping that digital downloads are updated with new languages as they get localized.