Thanks everyone !!
Ashwin - I followed what you said. I set application and data language to french, logged out and logged in again, created a report (I see all button names in french). Still I see 'ID - Description' as default at all places.
Raju - Sorry I don't understand how you want me to set member display as ID. I mean from where? Can you please explain a bit more.
Archana - I know we can get IDs on the report by selecting it from member selector as you explained. What I am trying to achieve is - I should not need to change it in member selector. By default member selector should show members by 'ID' (not by 'ID - Description').
Here is the screen shot of how I have set 'Context Defaults' from Admin. Please help me figure it out if I am missing any setting. We strictly need only IDs in our reports and it is very time consuming to change it everywhere manually.