japanese Keyboard Layout

Have you tried going to system prefs/language & text/input sources and making sure only the box for German is checked?

Otherwise you could try making a custom layout with

http://scripts.sil.org/ukelele

making sure you have chosen JIS as the Keyboard Type

I am a Maq user in Japan I and the Japanese use a Japanese keyboard that has roman and Hiragana symbols on the keyboard.

Go to menu bar -Open language and text preferences - input sources make sure ,keyboard and character viewer あQotoeri (sub menu ) ARomaji , あHiragana , アqataqana ɷ ( no symbol available) Ainu Qotoeri preferences -operation -input style choose Qana Typing then choose Romaji Keyboard layout what language I choose Finnish. 　

If you have US or Romaji language preferences and press Aち on Keyboard and then go to menu bar-menu tick あHiragana and type A ちit will be shown but Japanese set all their Qotoeri preferences to US or US international and Romaji Typing keys K 　a　 n　 a renders to 仮名 I use Kana Typing か and な renders to 仮名 for me it is easier.