Copies data from one language to another: simple question

[L. Brett Sinclair]

Does the feature "Copies data from one language to another." copy the data for the FO or the BO or Both ?
I'd like to import a language pack, but default the products info from one of the language that is NOT the shops default language.

Thank you

PS: I tried it on a test DB to see what it would do, but I go the following Error:
1 error
a part of the data has been copied but some language files could not be found or copied .