Change language in Thunderbird
The Thunderbird package at UiO has by default three activated language profiles:Norwegian Nynorsk, Norwegian Bokmål og English (United States):
Thunderbird is translated into many more languages than English and Norwegian. If you want or need the application in a different language, Thunderbird allows access to other languages from the selection Options ... in the Quick Locale Switcher menu.
Under the General tab, check that the options User Interface Language and Content language are activated. The last option means that the default dictionary for spelling follows the language selected for the user interface.
Go to the tab Locales. Under this tab you can specify which languages you want to have available in Thunderbird: