Method 2: Language Packs and Locale Switcher

The second method is useful if you wish to be able to switch languages on a regular basis. Here are the steps.

1. Install Thunderbird Portable to your portable drive
2. Start Thunderbird Portable
3. Download the Locale-Switcher extension and then install it by doing Tools - Add-Ons, Extensions tab and clicking Install within Thunderbird Portable.
4. Download the language pack you'd like to use and install it using the same method.
5. Close Thunderbird Portable, wait for it to stop writing to your drive, and then restart it to complete installation of the extensions.
6. Head to TOOLS and then LANGUAGES and select the language you which to switch to.
7. Restart Thunderbird Portable for the change to take effect (being sure it has fully closed before restarting).

Now, this method works pretty well and should work for just about everyone. The advantage is that this is a bit cleaner and you can easily switch between multiple languages. The disadvantage is that if your drop a local profile into Thunderbird Portable, you'll need to perform these steps again.