You could try the following, substituting in relevant dates in @d1 and @d2:
declare @d1 datetime = '3-jan-2000'
    ,    @d2 datetime = '2-may-2000'

select isnull(cast(nullif(case    when datepart(month, @d1)...

Have you tried adding

with (codepage = 'ACP')

to your bulk insert command?