Much of this is well known, but for people who want to have localized versions of Portable Apps this may come in handy:
Oh, btw, if you want to use another language than English, you should always choose multilingual
7-Zip Portable:
Choose your language in Tools -> Options
AbiWord Portable:
Choose your language in Extra -> Preferences
Audacity Portable:
Choose your language in Edit -> Preferences
ClamWin Portable:
I have no idea. I don't think it's available in other languages than English.
FileZilla Portable:
Choose language in Edit -> Settings -> Interface settings -> Language
Dutch users: geen Nederlands
Firefox Portable:
Extract the contents of the Firefox installation file and copy the files in localized to FirefoxPortable\App\firefox\
Gaim Portable:
Edit GaimPortable\Data\settings\GaimPortableSettings.ini bewerken.
Now change GAIMLANG. For example:
GAIMLANG=nl
GIMP Portable:
Edit GIMPPortable\Data\settings\GIMPPortableSettings.ini
Now change lang. For example:
LANG=en_US
LANG=nl_NL
Miranda IM Portable:
There are language packs available at http://addons.miranda-im.org/index.php?action=display&id=23
NVU Portable:
Extract the zipfile to NVUPortable\App\NVU\
Dutch users: klik hier!
OpenOffice Portable:
Install a language pack (http://oootranslation.services.openoffice.org/pub/OpenOffice.org/)
Sudoku Portable:
Sorry, as far as I know English only!
Thunderbird Portable:
The only way I know is installing a local version and copy the Program Files to ThunderbirdPortable\App\thunderbird\
VLC Portable:
Settings -> Preferences -> Interface
If you know more or if something doesn't work, just comment. I'll fix it