A group of fans have released a patch which adds English translation to Valkyria Chronicles 3, a tactical role-playing game Sega published in Japan only in 2011.
If you can get your hands on the Japanese version of the game, you can find the patch and rather complicated instructions on how to install it on your PSP or Vita, here.
The first game in the Valkyria Chronicles series was fully localized and released on the PlayStation 3 in North America in 2008. The sequel was also fully localized and released on the PSP in North America in 2010. Gamespot reviewed both games favorably at the time, saying their bold and fresh take on the strategy role-playing genre was both beautiful to look at and fun to play.
Unfortunately, in 2011 Sega confirmed that Valkyria Chronicles 3 would not be localized because the PSP platform wasn't popular and well liked in North America and Europe, and that Valkyria Chronicles II did not sell enough to justify localization costs.