How to match Japanese in spamassassin?

Solution 1:

Have you tried to use Mail::SpamAssassin::Plugin::TextCat (language detector)?
IMHO You should consider/evaluate it first.

header LANGUAGE_ZH X-Languages =~ /\b(?:zh)\b/
describe LANGUAGE_ZH Chinese language
score LANGUAGE_ZH 1.0

header LANGUAGE_JA X-Languages =~ /\b(?:ja)\b/
describe LANGUAGE_JA Japanese language
score LANGUAGE_JA -0.1

You can modify it to match "only one language detected/guessed" or some mixes of languages.

WARNING: Make sure the plugin is loaded by your SpamAssassin configuration.
It is configured in /etc/spamassassin/v310.pre file on Debian Linux.