Create a file called editor.js in /media/jce/js/ containing the following:

(function() {
	tinyMCE.onAddEditor.add(function (mgr, ed) {
		ed.onKeyUp.add(function(ed, e) {
			if (e.keyCode == 222 && e.shiftKey && e.ctrlKey) {
				ed.execCommand('mceReplaceContent', false, '„{$selection}”');
			}
		});
	});
})();

Then type in your text, select it, and use CTRL + SHIFT + " to create the quotes around the selection.

Or to use CTRL + SHIFT + 2

(function() {
	tinyMCE.onAddEditor.add(function (mgr, ed) {
		ed.onKeyUp.add(function(ed, e) {
			if (e.keyCode == 50 && e.shiftKey && e.ctrlKey) {
				ed.execCommand('mceReplaceContent', false, '„{$selection}“');
			}
		});
	});
})();

Delete the file I instructed you to create, and install JCE Pro 2.9.2 RC 9 - https://www.joomlacontenteditor.net/downloads/editor/pro/development

If you then have the site / admin language set to German, and you have the JCE German Language Pack installed - https://www.joomlacontenteditor.net/downloads/languages/editor/de-de - then CTRL + SHIFT + 2 will create the quotes on a selection of text, eg: „quote“

@SigridGR# - Please try JCE Pro 2.9.38 RC - https://www.joomlacontenteditor.net/downloads/editor/pro/development