I have carried out a clean install of Zammad on CentOS 7. The language defaults to Czech, I can translate with Chrome and set my lang preference under #settings/branding to English but it does not change.
It seems like your browser prefers to use “Czech” for displaying pages.
As I had testing something else I discovered that it’s not enough to simply tell Zammad it’s having another branding. This is caused by your browser. Sorry for taking so long for finding that out.
German visitors of your page will see the login in German and english ones in english.