Hi,
I just installed the portable version and wanted to ad an additional language for the spell checker. However it won't work the way it is described.
However I found a very simple way, so I want to share it:
1. Download the vocabulary language with your browser. You will now have a file with exention *.xpi
2. Extract the xpi file met 7-zip
3. copy the *.dic and the *.aff file to the folder: App\Thunderbird\dictionaries\
4. start Thunderbird and you can choose between the languages when you compose a message. Nothing else to do.
rgds, Wilfried