OK, so, on this topic we discovered that World Of Goo tries to open .en.ogg files whenever it loads. These don't exist, so it comes up in the debug output, but the names of these files are identical to names of the .ogg files it does load (such as "temp_main"), and which do exist.
This wasn't related to the problem in the thread, but I decided to see what would happen if I gave WoG one of these files.

I tried it out with an .ogg file that I renamed as temp_main.en.ogg, and put it in the same folder as temp_main.ogg. Lo and behold, next time I loaded WoG I heard that track on the main menu! What's more, when I changed the language in config.txt, the original .ogg file played again!

This is my theory: whenever WoG loads an .ogg file, it attempts to load its .en.ogg alternative. (For Dutch it will try to load its .nl.ogg alternative, and so on for every compatible language.) If it finds it, it will use that instead. Therefore, if you make a temp_main.nl.ogg file and load WoG in Dutch, it will play that instead of temp_main.ogg. That means that you can make language-specific sound files!
As far as I know, the language-specific sound file must be an .ogg file, only with the modified extension. And obviously, it must be in the same directory as the file you want to replace.

I think this will work with all sound files, even custom ones. This is because, as I said above, it attempts to load the language-specific alternative for every .ogg file it loads. The reason we only saw some of those files trying to be loaded in the debug logs is because when WoG starts it loads the resources.xml.bin file in the properties folder, and then temp_main.ogg for the title screen. However, when you open a level, it will load all files in that level's resources.xml.bin file, and therefore it will look for the language-specific file of every .ogg file it attempts to load, for example the level's music, or a custom Goo Ball's sound effects.

I hope that was followable. The last paragraph was mostly for an explanation for those interested, but the paragraphs before that should be enough to know how to use this awesome, unused feature.

By the way, I haven't actually tested all of this, as I said in the second paragraph all that I did was make a temp_main.en.ogg file. However I think my reasoning is fairly sound, so I have little doubt that this works for any .ogg file. If anyone feels like it they can check.