Replace query result with case/when failing
You don't mention what DBMS, but you probably need IS NULL instead of = NULL. However,  you're creating a complex case when this is a texbook use of COALESCE ():

SELECT
    "OEM_Description" = COALESCE( IT.[Description], I.[Description] ),
    C.Name as Customer_Name


Alternate solution based on comment:

IS NULL is not a function but a replacement for = NULL in your original like this:

SELECT
    "OEM_Description" = 
CASE 
   WHEN  (C.[Language Code] = 'ENG' and IT.[Description] IS NULL)
   THEN I.[Description]
  ELSE IT.[Description]
END,
C.Name as Customer_Name