I can not write with cyrillic letters on the signs. But I can write
on ukrainian (or russian) only in the chat and command block boxes.
LWJGL version: 2.9.0
Language settings: Українська(Україна) - Ukraine

• I wanted to write a sign on my native language (ukrainian), using cyrrilic alphabet.

• I can not write with cyrillic letters, I can use only latin alphabet.

What I did:
1. I put the sign.
2. I changed my keyboard layout to ukrainian (ctrl + left shift or ctrl + left alt).
3. I tried to write a sign on ukrainian.