A milk tea is a normal term for the type of drink they're likely talking about. It's different to a tea with milk.
This sentence doesn't sound right in English. There is no " English sounds unnatural" key in this particular sentence for reporting an error.
What in particular about the sentence doesn't sound natural? Is it the term 'milk tea'? To someone from the UK, it might sound odd as it's usually just referred to as 'tea'. However, in other areas, for example in Asia, the term 'milk tea' is often used to distinguish from teas with no milk such as green tea, jasmine tea, etc, which are far more common than tea with milk. For those who don't believe 'milk tea' is an actual term in use, see: https://en.wikipedia.org/wiki/Milk_tea
Most English speakers would simply say "tea with milk" both are correct, yes, but the answer given would be considered unnatural
"milk tea" sounds like you have a tea bag filled with pieces of dried milk.
Milk tea might sound strange to some native English speakers but it is quite logical when you think about it and in my opinion sounds OK too.
I actually wrote white tea, but yes tea with milk is a better answer. Milk tea does sound odd.
In that case you would say teh dengan susu or teh sama susu. Milk tea doesn't make much sense in Indonesian either I imagine.
It actually does. The way we make the tea is to seep the bag in hot milk instead of water.