Write some content that is valuable to others as a general resource. Right now your site simply tells visitors what you offer. As an example, add content about how a person might go about translating something on there own and the pitfalls they might encounter. Then find other webmasters and tell them why your content would be valuable to their visitors. I've got PR4, 5, and 6 links this way.
This is exactly right Try thinking from someone else's point of view, not your own. Why would anyone want to link to your site? Honestly. Give them a reason to link.
pr has nothing related with content. You need tons of backlinks which are related to your site's content or objective.
I dont know what niche you belong; but here is a search term you can use: allintitle:category + "PR 4"
Do off page optimization such as blog, forum posting, social bookmarking, directory, press release, article.
Search for some high pagerank dofollow blogs - you could easily find some PR4-6 internal pages and comment on those.
Pagerank is gained through the amount of backlinks you have coming to your website. There are many ways you can increase the amount of backlinks you have on your website, some primary and secondary ways. Primary ways, would be submitting to directories, search engines, blog commenting (make sure do follow). Secondary ways would be writing quality content which is that good, that others link to it, and tell other people about it. If you want to know more read http://www.back-links.org/How-to-achieve-a-pagerank.php