How can I hand a wchar_t* over to espeak without getting German characters spoken incorrectly?

I'm using espeak-ng to turn German-language traffic messages into speech. See this example text:

B6 Weserstraße B71 Seeborg vorübergehende Begrenzung der Breite. B213 Wildeshauser Landstraße Delmenhorst-Deichhorst wegen Baustelle gesperrt.

The espeak method call looks like this:

unsigned int spoken_message_uuid = 0;

espeak_ERROR Speak (wstring text)
{
  espeak_ERROR   error = EE_OK;
  unsigned int  *uuid  = &spoken_message_uuid;
  const wchar_t *input = text.c_str ();

  wcout << L"Speaking text:" << endl << input << endl;

  error = espeak_Synth (input, text.length (), 0, POS_CHARACTER, 0, espeakCHARS_WCHAR | espeakENDPAUSE | espeakSSML, uuid, NULL);

return error;
}

My issue is now the following: All the German special characters (ä, ö, ü, ß) are not being spoken correctly! Instead, something like A Tilde ein Viertel appears in the spoken text, as if UTF-8 text had been treated erroneously as ASCII.

Here are the respective versions of espeak-ng and g++:

pi@autoradio:/import/valen/autoradio $ espeak-ng --version
eSpeak NG text-to-speech: 1.50  Data at: /usr/lib/arm-linux-gnueabihf/espeak-ng-data
pi@autoradio:/import/valen/autoradio $ g++ --version
g++ (Raspbian 6.5.0-1+rpi1+b1) 6.5.0 20181026
pi@autoradio:/import/valen/autoradio $ apt-cache policy espeak-ng
espeak-ng:
  Installiert:           1.50+dfsg-7~bpo10+1
  Installationskandidat: 1.50+dfsg-7~bpo10+1
  Versionstabelle:
 *** 1.50+dfsg-7~bpo10+1 100
        100 http://deb.debian.org/debian buster-backports/main armhf Packages
        100 /var/lib/dpkg/status
     1.49.2+dfsg-8 500
        500 http://raspbian.raspberrypi.org/raspbian buster/main armhf Packages

espeak has been installed from Debian's buster-backports repo to replace version 1.49, which didn't work either. The voice I'm using is mb-de5.

OK, this is not exactly a solution, yet a mere workaround, but at least it works: I hand over a string instead of a wstring. The original string turned out to be UTF-8-encoded, so that all the special characters fit into a string resp. char* variable. Here is the adapted code:

unsigned int spoken_message_uuid = 0;

espeak_ERROR Speak (string text)
{
  espeak_ERROR  error = EE_OK;
  unsigned int *uuid  = &spoken_message_uuid;
  const char   *input = text.c_str ();

  cout << "Speaking text:" << endl << input << endl;

  error = espeak_Synth (input, text.length (), 0, POS_CHARACTER, 0, espeakCHARS_UTF8 | espeakENDPAUSE | espeakSSML, uuid, NULL);

return error;
}