When searching for an image, World of Goo will usually load filename.png. However, if there is a file called filename.es.png and the game is being played in Spanish, that image will be substituted. This works for all language codes.
In the original game, this is mostly used for image-based signposts and text such as "Island 1: The Goo-Filled Hills".
There is also a fature coded into the game for Profanity Pack users, which loads filename.profanity.png instead of filename.png, and properties/profanity.xml.bin instead of properties/text.xml.bin. As 2D Boy has not yet released the profanity pack, it is unknown how this feature would be activated.