They will select the (monolingual) dictionary using the language tag in the EPUB metadata.
It is not clear how they will let you choose a bilingual dictionary, probably by providing you with a new menu (with a firmware update).
What you suggest (ad hoc dictionary selection, based on the word --- e.g. a French word inside an English book) seems too complicated for this device. What about a word that might appear in dozens of dictionaries, like "pizza"? You should load all those dictionaries' indexes into memory to look for the word, there is simply not enough RAM for that.
The beta testing will probably assess the linguistic engine (how do you associate "applied" to verb "apply" ? or plurals to singulars, etc.) and the quality of the dictionary. They probably will use some freeware dictionary, like dumps from wikipedia. The French dictionary which is already included in these devices comes from a freeware Stardict created from a late-1800 French dictionary:
http://en.wikipedia.org/wiki/Diction...çaise_(Littré)