How can I translate default Wordpress Theme Strings?

Hi, I am translating a wordpress website but how can I translate the defaults fields that belong to the theme not set by the webmaster?
For example there is a "Search box", and I don't find any field for it?
And also there is an "Upload box section" and "Select the file to upload", these strings are automatically written by the theme, how can I change the Wordpress theme language to change all these strings at once?

Sorry it wasn't my intention to post twice, please remove this thread...