Hi. Where can I change the language for example: "Leave a comment
Make sure you enter the (*) required information where indicated. HTML code is not allowed." and so on.

i was looking to some documents. For example in the template language files and so on. But i havent found this kind of frontend words.

www.reinterpreten.de.

Hi. Each extension you are using in Joomla! comes with it's own language files. The specific language string you are looking for is part of K2 extension. Instead of messing with language files you can use the Joomla! language manager at "Extensions" -> "Language Manager" -> "Overrides" . Let me know if you need more help with this.