Directly after I posted this, I had the right idea.
- a boolean should actually be a boolean and not a string
- i copied the settings from the headings from the list website, but that contains invisible characters between the capitalized words (wtf)
- some settings are only supported on Chrome OS
The working config is now:
extraOpts = {
"BrowserSignin" = 0;
"SyncDisabled" = true;
"PasswordManagerEnabled" = false;
"BuiltInDnsClientEnabled" = false;
"MetricsReportingEnabled" = true;
"SpellcheckEnabled" = true;
"SpellcheckLanguage" = [
"de"
"en-US"
];
"CloudPrintSubmitEnabled" = false;
};