[Translators] Japanese translation update

[UTUMI Hirosi]

Hello,

I completed Japanese translation. It was so hard.
Please commit the attached file (ja.po.bz2).

Some thoughts:
1. Is it possible to split translation messages into Core and Plugins?
4000 messages are too big. Sylpheed has only 1771 messages.

2. Please update Translator's Information.
http://www.claws-mail.org/faq/index.php/Translator%27s_Information
> they should be sent to Paul at translations at thewildbeast.co.uk  
Maybe we should send them to this mailing list.
I sent this mail to translations at thewildbeast.co.uk,
but my translation was not committed.

Regards, 
Hirosi
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