When trying to rename items using non-latin characters they show up as question marks.
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Also true for accentuated latin letters (such as "é").
Edit : French characters now work for me (1.4.2). Maybe it was due to mods.
Works for me in Japanese letters, Windows 7 in Japanese, Minecraft 1.4.2 in Japanese.
Maybe it's locale that has to be changed for these namings to work
Please confirm that this still happens in the latest version.
While on a Lithuanian locale russian characters show up as ? marks, same with other non latin characters.
Maybe the system locale has to bes to the correct one for the characters to show up (i.e. if I turned on Russian locale it would show the characters), some reported that above.
I would imagine all of these strings are internally unicode, so system locale should not be an issue (though doing something intelligent w/r to CJK unification presents some interesting challenges)
Perhaps this is a font issue?
Confirmed in 13w02b.
Does NOT happen if non-ascii is edited into the NBT, only when it is entered in the anvil.
This leads me to believe it might be an input issue.
Confirmed for 13w09c.
After entering non-standard latin characters into the naming text box, the popup information box usually displays the incorrect question mark characters. (See screenshot "popup-incorrect.png".)
However, I just found out that you can get the correct characters to appear in the information box when you simply move the cursor in the naming text box with your arrow keys. (See screenshot "popup-correct.png".) This won't actually help with this problem though as the final item name will still contain those incorrect question marks. (See screenshot "renamed-item-incorrect.png".)
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This issue can especially be a problem for map makers (but other command block users as well) who use languages different from English and who don't use external NBT editing tools. The reason for this is that renamed command blocks with "say" commands will output those invalid question mark characters on the chat screen. (See screenshot "chat-incorrect.png".)
Confirmed for 1.5 pre-release.
This even happens with the language changed to what you're attempting to input (change language to Japanese, Japanese still fails to input properly).

Cannot reproduce in the latest version of minecraft