Languages
 

Is there a way to add/delete languages in the song edit window

Re: Languages
 

You can only change the category of the language how its listed, it wont actually change the language used in the song.
You can only select languages shown from the dropdown arrow, you cant add any others.

From the Songs/Edit menu select either Single of Multiples, select the Match items to filter on, fill in the search string, select and highlight the songs you want to edit.
In the language box use the drop down arrow and select the language you want to list them under. Then press apply changes.