How can I change the language of the QGIS Interface?

Sure, just go to the options dialog:

Préférences->Options->Langue (Settings->Options->Locale)

Check Forcer la nationalité du système (Override system locale) and set you language in there.

No need to uninstall QGIS.

All QGIS settings are stored in the registry at HKEY_CURRENT_USER\Software\QuantumGIS\QGIS\

If you look for the key HKEY_CURRENT_USER\Software\QuantumGIS\QGIS\locale you will see it has a overrideFlag and userLocale setting. Make sure that overrideFlag is set to false.

You can also start QGIS with a command line arg to tell it which language to use:

qgis.bat --lang en_US

Once you are in QGIS head to Settings -> Options -> Locale and make sure override system locale is unticked and that the additional info section gives you the correct language, mine is en_AU for example.

I found it... Options/Local then Override system local... :)