Display current date: UTF-8 issue?

Hi,

I use the following function to display the current date on my site:

function swg_gerdate_full() {
setlocale(LC_TIME, 'de_DE@euro', 'de_DE', 'de', 'ge');
$result = strftime('%A, %d. %B %Y');
return $result;
}

It is displaying the date in German. Worked so far, but unfortunately the current month March in German has an umlaut “?” (M?rz), and this umlaut is not being displayed in the browser, instead of it “?” appears.
Any idea how to solve it?
Or can I use a WP function to that I pass date parameters and it returns the current localized date?

Link: https://www.sw-guide.de/
On the top right you can see the date…

Thanks in advance,
Michael