Hi,
I'm not sure where your using the [LANG] token with the import if you have XML import files?...but usually the import token is used for the current langauge, otherwise with the CSV file update you can create a mapping to a csv field for the langauge.
However from what you say it seems your using the XML import export, In whcih case my guss is that you don;t have any model or product refs, the import system need these to match the import to existing or duplicated products/models, so make sure you have these in place. i'd suggest you do a qucik test with a sample set of product in a test system until you sure it's OK.
But if you have the refs setup and the same version of NB_Store for the export and import, it should be a fairly smooth process.
Dave.