@werdi, I'm afraid you 'sort of' solved it... You solved it for a visitor whose browser is set to German.
The issue appears when using the following combined setting:
http_accept_language: true
include_default_lang: false
This is a reasonable combination, so I tend to think this might be a bug in Grav.
With above combination, the language mentioned first in the supported list will always be ignored and replaced with the language set by the browser.
In your case, where de is mentioned first, German will be ignored when the browser is set to either English or Spanish and replaced with the browser's preference.
Solution (sort of):
Changing the value of either of the two settings will fix the problem, but then you will loose your preferred setting.
The bug:
I think this is an issue in Grav and not the langswitcher plugin, because without the plugin the same happens. In my default site, with your config and with browser set to Spanish the following happens:
- localhost/de/typography -> localhost/es/typograpy (= wrong)
- localhost/en/typography -> localhost/en/typograpy
- localhost/es/typography -> localhost/es/typograpy
I've logged the following issue #2948