Weird characters like ěščřž...€ get displayed by ? but that only happens if the tellraw command is in function. If it is in command block or chat, they work just fine.
How to reproduce can be seen in the video
MC-156448
Some characters are not displayed by functions but are displayed by command blocks
Resolved
Invalid
0
Unconfirmed
command, command_block
Minecraft 1.14.3, Minecraft 1.14.4 Pre-Release 4
Retrieved
Make sure you save the file with UTF8 encoding.