"Where is the longest border?"
Translation:Biên giới dài nhất ở đâu?
Why is there no qualifier before "biên giới" here? I thought that there should be one because it is referring to a specific border.
No the word border is just a general thing so it doesn't need to carry any classifier. The same is true for a word like đất nước/quốc gia (country/nation).