This might be too late for you, lerni, but may be it helps anyone else.
IIRC the Sitetree Importer sets the Title field only (and the parent id). Have a look in it's PHP source code, that part is easy. There, right where it sets the title field, set also the locale field. See the sitetree table(s) for it's exact name. (I write this out of memory only, forgive me for not adding more details)