Localization of WordPress blog language

I have translated my theme but still there are many words that doesn’t come from my theme but from WordPress itself, like short words from the blog interface such as “by: admin” and “comment:0” “post comment” and month names etc. There are more examples but I hope you understand what I’m getting at. The producers of the theme tells me that those words are generated by WP and doesn’t come from the theme itself. So how do I get to translate that? I have localized my theme with .mo and .po file after using PoEdit but these words never showed up in the translation list there. I’m always using English as my WP front-end admin language but the sites I’m producing is obviously in other languages.