The first time you start Monkey Island SE, you can choose the subtitle language, but after that you can’t do it anymore with the game interface.
You can follow the instruction on the steam support page or you can use my little tool and if you don’t trust me the source code is available.
Have fun !
Whaly
is now in my bookmark and in my rss reader, as it should be in yours 
– From Theory… To Practice! –
