How to set the language of a doc to a DB field
I think I found a work around for you.


Set up a new dataset to pull back the language value (let's say language_dataset into column language_value).
Create a parameter, let's say called language.  Set it to internal and set its default value -> Get values from a query and use the dataset and value you just set up.
In the Report properties, Language -> expression and set the expression to =Parameters!language.Value.


Let me know if you need more details.