language italian for version 3 beta 5

The beta is only in english. You have to use Method 2 from here to change the language. But I don't know if this Addon works with FF 3 beta 5.

As I said to Laszlo, you can do like this:

1. Download and install language pack for 3b5 in here.
2. Create a text file called FirefoxPortable.ini on the root of your Portable Firefox install with the following content:

[FirefoxPortable]
FirefoxDirectory=App\firefox
ProfileDirectory=Data\profile
SettingsDirectory=Data\settings
PluginsDirectory=Data\plugins
FirefoxExecutable=firefox.exe
AdditionalParameters=-contentLocale Language code you downloaded - ex: it
LocalHomepage=
DisableSplashScreen=false
AllowMultipleInstances=false
DisableIntelligentStart=false
SkipChromeFix=false
SkipCompregFix=false
RunLocally=false