The answer is simple: The game is still sold in its original form complete with only a Japanese interface, Japanese audio and Japanese subtitles on DLSite (Japanese page | English page).
The original Japanese version was meant for sale to native Japanese speakers, students of the Japanese language and Japanophiles (Those devoted fans who willingly pay a premium to have an imported, original Japanese anime DVD box set made in Japan instead of an English translated, equivalent that's locally released by ADV, Funimation or some other video redistributor). In order for the translated version to be permitted to be sold outside of Japan and not be in direct competition with the original product, the Japanese interface and Japanese subtitles were excluded. The dev probably wanted to expand the H-game's audience (and sales) without the process cannibalizing sales of its original Japanese product.
If you really want the missing Japanese interface and Japanese subtitles then you'll have to buy the original product from DLSite or try to find it elsewhere over on the 'Net. There are fan translated versions of the original version on the 'Net but usually the Japanese support (interface and subtitles) is removed (It may still be intact for the game's main menu - It depends upon the fans who translated the game) since they aren't necessary with the English or other language translation in place to make the game playable for non-Japanese speakers. Also, the translation in a fan translated version might not be on the same level as a professionally translated (by linguists), retail version.
This is the GOG version of the game.
The original Japanese version was meant for sale to native Japanese speakers, students of the Japanese language and Japanophiles (Those devoted fans who willingly pay a premium to have an imported, original Japanese anime DVD box set made in Japan instead of an English translated, equivalent that's locally released by ADV, Funimation or some other video redistributor). In order for the translated version to be permitted to be sold outside of Japan and not be in direct competition with the original product, the Japanese interface and Japanese subtitles were excluded. The dev probably wanted to expand the H-game's audience (and sales) without the process cannibalizing sales of its original Japanese product.
If you really want the missing Japanese interface and Japanese subtitles then you'll have to buy the original product from DLSite or try to find it elsewhere over on the 'Net. There are fan translated versions of the original version on the 'Net but usually the Japanese support (interface and subtitles) is removed (It may still be intact for the game's main menu - It depends upon the fans who translated the game) since they aren't necessary with the English or other language translation in place to make the game playable for non-Japanese speakers. Also, the translation in a fan translated version might not be on the same level as a professionally translated (by linguists), retail version.