Please have a look at [http://kb.mozillazine.org/Change_Default_Mozilla_Language this article] for several ways to accomplish this. Apart from installing a language pack, you will either need to switch the preference ''general.useragent.locale'' to en-US in Thunderbird’s Configuration Editor also known as about:config, install a locale switcher add-on (that’s kept up-to-date) doing it for you, such as [https://addons.mozilla.org/thunderbird/addon/simple-locale-switcher/ Simple Locale Switcher], or specify the locale code as a command line option to use in shortcuts. You could also choose to combine/use both methods.

Thank you, Tonnes! Perfect solution. K.r. /hans peter