Your system locale is not Japanese, that is the reason why the game crashes. There are two solutions to this problem:
- Switching your system locale to Japanese.
- Installing Applocale.
I suggest you use the second option, though. Applocale is a program that runs the program you chose in the locale you selected, without changing your system locale and rebooting your PC. It's essential to play Japanese games that were not officially localized, like .flow, or various fan-translated games. You can find the download link and installation instructions here. To run .flow with Japanese locale, select "日本語" (Japanese) in the "Language of the application" list, it should be somewhere at the bottom.