Well, to change the actual startup language you can use a tweak tool... I use Cocktail. There is also a freeware thing called OnyX, but I don't know if it has that same option.
If you just want to input japanese text, the input selector under International in the OS X settings works very nicely.
In Tiger, all you need to do is go into the International applet in the control panel and change your language preferences. Drag Japanese to the top of the list.
In Tiger, all you need to do is go into the International applet in the control panel and change your language preferences. Drag Japanese to the top of the list.
ProTip: In addition to keyboard shortcuts for tabbed browsing, shortcuts for the bookmark bar can make you a more efficient Safari Internet Surfer. Command + 19 activate the first nine non-folder bookmarks in the bookmark bar. You can use these keyboard shortcuts even when the bookmark bar is hidden (toggle with Command + Shift + B), freeing up screen real estate without losing quick access to your most frequented bookmarks.
There are also some nifty plug-ins like SAFT which allow you to do all sorts of crazy stuff like drag your tabs around and such. Some are shareware tho.