This is a potential duplicate of this and for that I apologize, but new information has presented itself so I think the question is justified.
There now exists a /ja/ subfolder and a SourceTree.resources.dll. This hints that there might already be something in place. I couldn't find anything during the installation or in the config file (SourceTree.exe.config).
Then I also found this repo which seems suspiciously related, and looks to be an almost completed translation.
If this is true, is there perhaps a way to testrun this localization? A config change or a command line switch maybe? Please excuse my bluntness. I'd kill for japanese localization!
Thanks for any help!