I have installed WordPress on new site, I need it in Czech language. There is Czech translation available. But there is no theme respecting localization idea at all. So after translation and fixing in localization file I went through all theme files searching for text to be translated again. Especially files like "single.php" and "page.php" drive me crazy, there are same text to be translated, "comments.php" very similar. Themes also do not respect format of date and time chosen in WordPress settings page.
And if I am right, there are also some English text in WordPress code itself.