Unfortunately what mivo said is not 100% correct.
I agree with mivo that this is the normal behaviour. Unfortunately I had already one different expirience and this was with the german "Fallen Enchantress - Legendary Heroes" Edition.
There I had no possibility to switch to the german version until I bought the german retail version. After activating the key the Fallen Enchantress I already had was replaced by a Multilingual version (german / englisch) which you cannot buy at steam!
However this happened only once for all of my 450 Steam games - but the game was from Stardock...
I suppose it depends on the publisher....
Since I am interested in the Founder Elite Edition I would like to see an official Statement from Stardock that other languages will be supported.