The Japanese mobile game market is a lucrative one, capable of making game publishers the world over salivate. But it is rarely an easy space to break into. Finland’s Supercell has managed to find a loophole of sorts though, collaborating with GungHo Entertainment to make Clash of Clans a hit. And now, just after news of $1.5 billion in investment from SoftBank and GungHo, the company’s other prominent title Hay Day is finding success in Japan, becoming as the top free app last Friday (see chart below) after the Japanese version hit the App Store. Hay Day has also been spotlighted in the ‘featured’ section of the Japanese App Store, and it has been heavily promoted by the folks over at Japanese app review site AppBank as well. Readers may recall that Supercell also credited part of the success of Clash of Clans to AppBank’s promotional efforts as well. The GungHo/Supercell partnership is intended to be a two-way street, so it will be interesting to see how Supercell can help promote GungHo’s game titles in European and North American markets. Puzzle & Dragons just launched in the UK market on October 16, so lets wait and see what kind of…