Earlier this week Idea Factory confirmed it would be localizing Monster Monpiece for North American and European release later this Spring, albeit in censored form due to the "strong sexual nature" of certain artwork in the upcoming Vita card battle game. Today the publisher saw fit to further address the reasoning behind its decision, as well as the effects: as a result of the censorship, approximately 40 out of the 350 total "Monster Girl" card images in Monster Monpiece will be removed from the non-Japanese releases. In its original game announcement, Idea Factory provided an exact list of the Monster Girl "evolutions" which would be affected.
Note that the removal of artwork will have no effect in the ability to "level up" said Monster Girls for the localized release (their cards simply won't change in appearance compared to others which were not subject to the censorship), or any other gameplay features for that matter. So why the rub? Apparently some of the Monster Girls are depicted in a manner that might bring about the wrong sort of attention to Idea Factory's international business:
"...Western society is not as lenient as that of Japan when sexual images are involved—especially images of humanoids that appear to be younger than a socially acceptable age. The borderline of what is “acceptable” will always be extremely gray and vary from person to person, but as a responsible company working in the U.S., we had to make the difficult decision that we did. We sincerely apologize for those who do not agree with any level of censorship, but we greatly appreciate your understanding with the decision we have made."
Meanwhile, Idea Factory explained that the difference in age ratings betweeen the North American and European releases of Monster Monpiece is simply a result of how the each territory's respectiive ratings authority judged the game's content matter, based on the sample assets provided by the publisher. Sounds obvious, but the materials showcased to both the ESRB and PEGI were in fact the same. The ESRB deemed Monster Monpiece to be for Mature audiences only, while PEGI settled on a 12+ rating due to "minimal amount of violence."
Idea Factory's Facebook update has its statement in full.