[Risolto] Custom langauge

As our main language is dutch and we want to make a belgium language added we have 2 options for language available : be-fr and be-nl. Automatic translation from default language (nl) to (be-nl) doesnt work. But copying and pasting every page and string manually is a lot of work. How can i automatictly get the (be-nl) website to work in dutch language (same as default)?