Translation:Those white birds fly up the tall mountain.
I said "Those white birds fly onto the tall mountain" and was marked incorrect.
"Those white birds fly up to the tall mountain." is ambiguous in English. It is correct, however, "up to" can also mean "as far as"...
If you were just given the English sentence "Those white birds fly up to the tall mountain." (as per the correct answer was given when my answer was marked incorrect) then it is unclear if you mean the birds are flying up onto the mountain, or if they are flying as far as the mountain.
That is, it would be unclear if the correct Hungarian translation would be "Azok a fehér madarak felrepülnek a magas hegyre." or "Azok a fehér madarak felrepülnek a magas hegyhez".
Maybe next time I will try "Those white birds fly up onto the tall mountain" and see what happens.
Raphi_K is right. One walks up a mountain, runs up a mountain, climbs up a mountain and even drives up a mountain. These actions are done on the ground. But one (especially a winged creature) does not normally fly up a mountain; one flies onto a mountain. Given the Hungarian sentence (... hegyre), the best English translation appears to be, as Raphi_K says, "Those white birds fly up onto the tall mountain".