I FIND A WAY !
If you want to select a language that is not listed in the steam properties, but that is listed in the game folder "locale".
There is a config file, config.js, in the game folder
For the french language for example :
Lign 36 in notepad++ :
var FORCED_LOCALE = null;
var FORCED_LOCALE = 'fr'; (don't forgot punctuation)
Anyway i offered my help to complete the translation of the game to french.
I heard there was a partial French translation of the game during the beta.
I see there is a "FR" folder in the game folder.
But i don't find anyway to select French language, i don't see it in steam properties.
I see other language in the list of the steam game properties
I can help you to finish to translate the game if you need.