German umlauts (ä, ö and ü) not properly displayed and spoken.

[Binozo]

Hello,
there is an issue with German umlauts.
When you send a response with ä, ö or ü, Google Assistant will not pronounce and display them correctly. For example, "ü" is displayed and spoken as "ü".
I found no information about this and I found nothing in the settings.

I tried to use the right charset for this.
String response = new String(responseBody.getBytes(StandardCharsets.UTF_8));
And the Text is displayed correctly in the console. (When I debug it locally)
{"fulfillmentText":"Diese Funktion wird derzeit noch nicht unterstützt...

Before I set the charset:
-> {"fulfillmentText":"Diese Funktion wird derzeit noch nicht unterst�tzt...
After I set the charset:
-> {"fulfillmentText":"Diese Funktion wird derzeit noch nicht unterstützt...

But the issue is, Google Assistant doesn't receive the Text correctly.

Any ideas how to fix this?

[Binozo]

Update: This worked locally but not on production:
resp.getOutputStream().write(new String(("" + responseBody).getBytes(StandardCharsets.UTF_8), StandardCharsets.UTF_8).getBytes());

The biggest pain in the ass I've ever had.

[Binozo]

I removed these lines
resp.setContentType("application/json; charset=UTF-8");
resp.setCharacterEncoding("UTF-8");

and it started working.

The Problem is, ä, ö and ü is UTF-8.
This is ridiculous.

[Binozo]

Still same issue