Certain letters not common to the English language are not able to be correctly displayed by functions, instead showing a question mark.

Steps to reprouce:

1. Create a function.
2. Inside that function, write out commands that display characters not used in the English language. Examples are included below.
3. Run that function inside your world. You will see that the characters are not being displayed properly.

Example commands:

tellraw @a [{"text":"á","color":"red"}]
say ñ
give @p diamond_sword{display:{Name:"{\"text\":\"æ ç\"}"}}

Attached is the result of those three commands being executed in a function.