I had a report with 6 parameters. One of them, it happened to be Language, had a default value of 1 in the parameter window. The rest had values from datasets. Once deployed, if you chose a data driven parameter, the language default would disappear and ask you to select a value. This was very inconvenient, as it was a hidden parameter in this report.
After googling and looking in the forums, I realized this is an ongoing issue in SSRS. So I had to come up with a workaround.
I played around with it for a while, and figured out a way to make it work. I created a dataset called DefaultLanguage, with the SQL:
Select 1 as IDI then set the default for the language parameter to that dataset.
Now when any other parameter is changed, the default language stays selected!