From Online Manual
Why Is My New Thread Indicator Missing
When using a language other than English, the new thread indicator might be missing. This is because you have installed a custom theme after installing a language. This will cause the indicator to be missing in the new theme.
To solve this problem, in your forum's directory, go to Themes > Your current theme > English and copy the two files (index.php and new.gif). Return to your theme's folder and create a folder (if it has not yet been created) and name it according to the language that your forum is using. For example, if you are using Spanish_es UTF-8, create a folder and name it "spanish_es-utf8". Finally, paste the two files into the folder.
If you want the indicator to be displayed in your preferred language, you can make your own indicator and place it in the folder you created previously (in this case spanish_es-utf8) within your theme's images folder and name it "new.gif".