Hi,
i am trying to add german umlauts: "öäüÖÄÜ".

I have seen a "solved" thread with the same issue.
https://umod.org/community/kill-feed/4164-special-characters-not-showing
But OP did not deliver how he solved this -.-

In KillFeed.json:

"3.7 Allowed Special Characters": [
      ".",
      " ",
      "[",
      "]",
      "(",
      ")",
      "<",
      ">",
      "ä",
      "Ä",
      "ü",
      "Ü",
      "ö",
      "Ö",
      "ß"
    ]​

But this has no effect.
I even edited the KillFeed.cs plugin itself:

// initialize allowedCharacters and populate initial allowedCharacters population
            allowedCharacters = new bool[65536];
            for (char c = '0'; c <= '9'; c++) allowedCharacters[c] = true;
            for (char c = 'A'; c <= 'Z'; c++) allowedCharacters[c] = true;
            for (char c = 'a'; c <= 'z'; c++) allowedCharacters[c] = true;
			
			allowedCharacters['ä'] = true;
			allowedCharacters['Ä'] = true;
			allowedCharacters['ü'] = true;
			allowedCharacters['Ü'] = true;
			allowedCharacters['ö'] = true;
			allowedCharacters['Ö'] = true;
			allowedCharacters['ß'] = true;

But it doesn't help :(

It always schows a "?" Questionmark instead of the special charakter.

I'm not sure that Rust supports those symbols anymore, but you can try saving as UTF-8.

I already saved it as UTF-8 and uploaded to the server.
Once reloaded the plugin, it changes all the umlauts back to "?" in the file.
Why would Rust not support special characters?

Merged post

Ok i solved it.
I opened both the plugin.cs and the .json and had to explicit convert them to UTF-8-BOM in notepad++.
I had to edit lots of broken characters and had to replace the "_formattingChar".
Not it works fine :)