That is a test article (You need to create a article with more words and when you click in that button, is going to open the full article)

Did you switch to your language in the settings already?
If so, you have to find the translation file of the plugin or theme the button is from. Should be the theme, but I'm not a wordpress expert. There are (or at least should be) folders named "languages" in the plugins/your-plugin or themes/your-theme folders. You have to edit the translation with a po file editer, I use poedit, but you may find better alternatives.
If the button is from some theme or plugin that doesn't have the languages folder, then the developers thought themselves "speak english or die" and you will have to edit the code. Or, like I would, look around for alternatives.
Hope that helps.