ok, settings seem to fit, however, when I look into the source of your calendar page, I cannot find this entry:
<div id="cfgLocale" style="display:none;">de</div>
which should have been added by the page template, based on your settings.
are you sure your calendar page uses the correct template calendar.html.twig from the plugin ?
the relevant part of the template is this:
{% set lang = grav.language.getLanguage %}
{% if page.header.lang|length > 0 %}
{% set lang = page.header.lang %}
{% endif %}
<div id='cfgLocale' style='display:none;'>{{ lang }}</div>