Sakura Dungeon is an After the End setting and the world is Earth in the future
Many, many comments by Yomi imply that this world is really Earth All Along sometime in the far future, when society has regressed to a medieval setting and magic has become common-place (again). Yomi herself is described as ancient and she also was sealed for an unspecified amount of time, making it unclear when it happens, but things like the many different clothes (such as the Playboy Bunny outfit which came "from a very interesting country"), references to boxing as a sport, Yomi's prefered architecture and interior design being blatantly Japanese ("from her homeland") or a camera showing up among Charlotte's various products make it fairly obvious that something happened. Even the fox spirit aliens might be from Earth originally and simply forgot about it, since they talk and write in a language that Yomi and even Ceri can understand. They also use the exact same combat skills as Yomi's party, explained as Nanomachines.Yomi doesn't just resemble the fox-like aliens, she is one.
Sent to Earth for some reason: a Superman-style escape from the dying planet, or as an advance scout, perhaps.