lang=... is actually supposed to work. it-CH isn’t supported by LT, though (use simply it). If you could provide a full test file, we’ll try to have a look.
Thank you Daniel for your answer. It works to detect the “main” language code. In our company we use LT premium for our customer care employees. They are answering e-mails for Germany, Switzerland and Austria in our ticket system - it would have been great if the browser plugin (Safari or Chrome) would automatically set the variant of the German language by defining lang=“de-CH” in the textarea for our reply.