Most people can use dmm.com (all-ages) with a copy/paste edit to a cookie, which you can find on flowerknight.wikia.com/wiki/Tutorial:_Get_Started though that may have changed in recent months.

Nothing is required for most western countries to play on dmm.co.jp (R-18)

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There's no permanent lockout or anything if the site realizes you're a dirty foreigner. Try logging in with the language set to Japanese. If that doesn't let you see the game, try the cookie. If that doesn't work, it gets more complicated.