From Online Manual
Why is my New Thread Indicator Missing
When using a language different than English, the new thread indicator might be missing. This is because you installed a custom theme after installing a language. This will cause the indicator to be missing in the new theme. The image has to be added to the themes images folder in a folder by the name of the language that is manually created.
To solve this problem, in your forum's directory, navigate to Themes > Your current theme > English and copy the two files (index.php and new.gif). Then, come back to your theme's folder and create a folder (if it is not created already) and name it with the language your forum is using. For example, if you are using Spanish_es UTF-8, then create a folder and name it "spanish_es-utf8" and paste the two files into the folder.
If you want to have the indicator in your own language, you can simply create your own indicator and place it in the folder you created previously (in this case spanish_es-utf8) in your theme's images folder and name it "new.gif".