It stays Japanese
However, if it didn't have a nickname and had its Japanese name, it would change to an english name. So, let's say there was a Pikachu named ピカチュウ, which is its Japanese name. If you took a thunder stone and evolved it, it would be named Raichu. But if the Pikachu had a name, like, let's say it was called サンダー ("Thunder"), when it evolved it would keep its name.
It would still be Japanese, and have the little icon. You would still be able to do Masuda's Method with it.