I got index.erb and index-de.erb, and somewhere I saw an example where this resulted in index-de being rendered when I18n.locale was :de, but i just tried and it did not work and I could not find any documentation on this subject. So does this actually work or do I need an extra plugin for this ?
Localization is built into rails as of 2.2; you'll probably want to check out the i18n screencast and/or the guide.
Edit: in particular check out the guide's section 3.3, "Localized Views". The short answer is that the correct filename is index.de.html.erb.
they must be named index.LOCALE.EXTENSION so e.g. index.de.erb or index.de.html.erb for de locale (I18n.locale)