Thanks for your reply.
The pages that disappeared: The pages linked from the main menu, I've changed the names of these menus (portal, etc.).
So it seems that you are right and the problem is in new "language files".
Where are these files residing?
Can I just overwrite them with ones I got in the backup?
You say "many pages" - what pages exactly - and which ones can you still
Can you find the pages if you search for them?
Can you check to see in the database if the page data is still there?
If positively, it might just be a matter of old links being loaded from the
language files again, replacing some of your customization.
If negative, something worse may have happened - and you may need to
manually SQL in the missing data to the database.