In my opinion, translating all content is stupid. On the other hand, there is some content which is clearly project-specific. For instance, I recently needed to create an article on the history of Georgia (US state). The obvious route I have chosen was to translate an English article first, and then to add the content I find in addition. I think it is kind of obvious that articles on en.wp on the history of US are of good quality. As another example, articles on Paris metro stations on fr.wp are not fantastic, but they are still a good basis to improve on after the translation; on the other hand, en.wp articles on the same topics are often substandard and not really recommended for translation in other projects.