replace default Dutch Thunderbird by en-US version

Of course, the default version of an application will be determined by the language of your desktop. If you do not wish to change your default language, you can change the language ("locale") of one specific program:

env LANG=en_US thunderbird

Of course, the locale concerned must be installed on your system.

To automatically launch Thunderbird in the specific language through your regular user menus, adjust the .desktop launcher file for Thunderbird.

1. Copy the system wide thunderbird.desktop file to .local/share/applications under your home folder. With a terminal command, it goes like:

   cp /usr/share/applications/thunderbird.desktop ~/.local/share/applications/
   

   Of course, you can use your file manager to do this if you prefer. .local is a hidden folder, which you will see only if you turn on Show hidden files in your file manager.

2. Edit the .desktop file you copied with your text editor and change the existing line

   Exec=thunderbird %u
   

   to

   Exec=env LANG=nl_NL thunderbird %u
   

3. Save and close the file.

Your menu system will very quickly pick up the change. Next time you select Thunderbird in your application menu, it should launch in the new language. Other users of the computer will still have Thunderbird in their default language.