I found a tool called "ResEx ... the composite, translation friendly .NET Resource editor" which can be found here that meets exactly this requriements. It provides a nice spreadsheet interface which makes it also for non programmers easy to translate the resources in the files. It is also possible to group resources together by using an underscore e.g. Menu_Home, Menu_Customers; they will all be shown under the tree node "Menu" which makes it easier to understand where the translations are used. Even adding and deleting localization languages is very easy.
The only thing which is missing is to have an automatic translation by using one of the several online translators available nowadays. I will suggest it on the forum and maybe this feature will be available in a future release.
Another very useful tool when working with resource files is the Resoucre Refactoring Tool on CodePlex. It helps you by localize strings in your source code by putting the string into the resource file and replacing the corresponding call in order to get a translated string.