On the download page of PTB, there is a link for getting Thunderbird in other languages, which is this link here:
https://portableapps.com/support/thunderbird_portable#localization
At that location, there is a link to dowload an Aviary language pack, but I could never get to the location that the link specifies. I Recomend changing the link to:
https://portableapps.com/apps/internet/thunderbird_portable/localization
This page gives very easy instuctions and links to the correct files where you can change the language on your PTB. It gives instructions on how to have different languages installed at one time, and then you can switch to whichever one you want. It worked fine the first time and didn´t have any trouble with it, as they install as extentions.
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you will need Locale switcher (https://addons.mozilla.org/thunderbird/356/) and the .xpi file that matchs your version (change version number in this link: http://releases.mozilla.org/pub/mozilla.org/thunderbird/releases/1.5.0.7...)
then select your language, download and follow instructions in the Locale switcher extension homepage.
Hope this helps- Regards.
Sergio