Translation:Kékestető is high whereas Lake Balaton is deep.
"Tall" is OK here. I think it's mostly a matter of personal preference. Not that this is a perfect gauge of such things, but "tallest mountain" and "highest mountain" both get about the same number of Google results, and there are plenty of examples of fluent English speakers and writers using both.
It would be good for the exercise to accept both words, though, so it's worth reporting if you get the chance.