A few minutes ago, YouTube's S60-native application got upgraded to v2.2.1, bringing official support for 11 countries (up from 5 in previous versions) - it'll now work in Australia, Canada, France, Germany, Ireland, Italy, Netherlands, New Zealand, Spain, UK and the USA and, crucially, will work in French, UK English, Italian, Spanish, German and Dutch. The SISx file is designed for all S60 3rd Edition phones. To get YouTube/S60 v2.2.1, go to m.google.com in Web.
If you know the trick of extracting the SISx file, it does install on S60 5th Edition phones as well, with some caveats about not using the full-screen for video playback. We're still waiting for a full nHD-optimised version of the application.