If I understand your question correctly, i don't believe it can be done in a SP.
However [As you probably realise] you could dynamically build the query in the application.
Example in VB
Dim sSQL as String, sWhere as string
sWhere = "where lang=@lang"
sSQL = "if @param=1 begin select country from countries " & sWhere & " end if @param=2 begin select name from customers " & sWhere 'etc etc