As described in the Toolbar topic, the NLS Description 
button opens the Translated Descriptions window.
Here you can enter multilingual translations of the short description and the description of the record from which this window was called.
To enter a description for a new language, select the language from the LOV in the Nls field. All languages defined on A.10.01 Languages are available. Next, enter the translated descriptions in the Short Desc and Description fields and save the new record using the Save button (F10). The translated descriptions will be displayed on the screens depending on the login language.