Many thanks to @Kotaku for tweeting their link!
This post focuses on the lines that Keanu spoke in Japanese. They said the following about Keanu:
Reeves is well known to be a hard working actor – say what you will about his acting quality, the man takes his preparation seriously. For his upcoming movie, 47 Ronin, a movie based on a Japanese legend, Reeves worked under a Japanese coach so he could perform his lines in Japanese as well as English.
Keanu said the following on the matter:
“I recorded every scene with Japanese dialog according to the wishes of the director. If the director wanted to, he could probably release a full Japanese version of the film.”
The article is here.