Signatures support the use of URLs (they will be clickable), WP's emoticon set and plain ASCII text only, which would discount the use of Japanese or any other characters relying on encoding systems such as Unicode.
As an aside: the information about the signature in your profile settings is a little out-dated in that HTML formatting cannot now be used - I believe the restrictions on images and scripts etc were imposed after that information was written and probably as a result of it, since HTML would normally allow such things.
Questions about the signature's use of images would have been easily resolved by checking WP's rules - which should have been read before making any posts here:
WP rules wrote:
Signatures, Avatars, and custom ranks.
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* To use a signature, we ask that you place the signature text in the signature form on your user profile. Including signatures in the actual text of your posts is not permitted. WrongPlanet moderators may edit any post that contains a signature in the text of the post.
* Signatures should be no more than 6 lines long and may not contain any images or scripts of any kind.
* Please keep to the rules for avatar size, which can be found at the bottom of your Profile page.
* The forum software will allow you to define a custom rank once you have accumulated 1000 posts.
Link to WP rules:
http://www.wrongplanet.net/postt12459.html
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Giraffe: a ruminant with a view.