Updated Version

I've still the Notice messages : 

https://prnt.sc/ubohg5

This is not secure, because someone could use this the explode or something like this.

Hey Notices are not any threat they are just notices some variables are empty. You are on localhost where the notices are visible. But you can fix them.

edit lib >> includes >> functions.php and find the function get_option 

and replace with

function get_option($option_name) { 
		global $db;
		$query = "SELECT * from options WHERE option_name='".$option_name."'";
		$result = $db->query($query) or die($db->error);
		$row = $result->fetch_array();
		$count = $result->num_rows;
		
		if($count > 0) {
			$option_value = stripslashes($row['option_value']);//this will remove database slashes from values
		} else {
			$option_value = FALSE;
		}
		return $option_value; //This function returns option value.
	}//get option value function ends here.

Thanks. My last Problem i've is the translation is not working. I found misspelling in german and I tried to fix it but this isn't possible for example :

login_script.po German line 1145 - 1146 =>

msgid "Dashboard"

msgstr "Instrumententafel"

Changed to =>

msgid "Dashboard"

msgstr "Dashboard"

Saved it, cleared cache, restarted XAMPP and loggedin again, but still shows : Instrumententafel instead of Dashboard.
What I'm doing wrong?

Edit locale\de_DE\LC_MESSAGES

login_script.po in poEdit software make your changes and save it so .mo file can also update translation happens in .mo file which poEdit would produce from .po editing. 

Okay, so when I wanna add something new, how is this possible? Just add to .po and save it again?

In that case you would need to first add string in PHp file and run updater to update main .pot file. And from that .pot file you would have to update your .po file and then .mo file if you are running single language website then you can use simple strings instead of translateable strings.