Print

Q: how can I change default client content language?

A: It is possible to achieve this by modifying the web.config file.
You should add the contentLanguage attribute to the shell site in the section (it is shown in bold below):

<site
        name="shell"
        virtualFolder="/sitecore/shell"
        physicalFolder="/sitecore/shell"
        rootPath="/sitecore/content"
        startItem="/home"
        language="en"
        database="core"
        domain="sitecore"
        loginPage="/sitecore/login"
        content="master"
        contentLanguage="da"
        enableWorkflow="true"
        xmlControlPage="/sitecore/shell/default.html"/>

Now your default content language should be changed to Danish.