Cutting and pasting of Japanese text works just fine in the edit box, but cutting from the channel/etc. window has a very high failure rate, resulting in a string of random characters.
Sometimes it works, but more often than not it doesn't. There seems to be some rules that govern when it will and won't fail, as trying to cut&past the same characters gives the same result each time.
End result, I often have to cut & paste random chunks of a line multiple times before I get one that has the part i need without being turned into gibberish.
Example, I have previously typed this line, and hit enter, so it's in the channel window: とても面白い
If I cut and paste the 面白 part by itself, i get this when I paste it: る擇逋ｽ
Cutting & pasting the ても gives 縺ｦ繧る.
But cutting & pasting the full phrase works perfectly, as do some other combinations of characters within the phrase.