Translation:This motorway lies between Budapest and Lake Balaton.
Why "a Balaton" translates to "Lake Balaton"? Could someone explain please? Thank you.
On the one hand, a Balaton is how it is frequently called in Hungarian. It is such a significant natural feature that it gets referred to just with the definite article. On the other hand, in English, we very rarely use "The" with the name of a lake. In English, we call it "Lake Balaton." So the answer is: just because that is what this place is called in each language.
I often call it / hear it called "the Balaton" by my colleagues, but that might just be my Frenglish and their Hunglish :P
'Road' was not accepted for 'autopalya'. But in American English, we use 'road' and not 'motorway (which is British English).
It's more specific than "road" though; "highway" would be a better American equivalent.