char is always UTF-8 codepoint and therefore exactly 1 byte.
wchar is always UTF-16 codepoint and therefore exactly 2 bytes. dchar is always UTF-32 codepoint and therefore exactly 4 bytes;'Ğ' has the value U+011E which is a lot larger than what 1 byte can hold. You need 2 chars or 1 wchar/dchar.
https://unicode-table.com/en/011E/