Automatic language selection for Thunderbird package

Hi sissbruecker,
I added support for automatic language selection in your thunderbird package. Most likely a user is to have the same language for Thunderbird, as it has already for Windows. This script checks the current system language and downloads the right installer.
It would be great if you accept my pull request.
The problem with your previous solution was that on every update thunderbird gets overwritten with the en-US language. That is not optimal. With my update this doesn’t happen. I have tested it.