Complex sentence: Lángos is sold at many fast-food restaurants not only in Hungary also but in Austria, where it is, especially in Vienna, very popular as a fast food on fairs and in amusement parks like the Prater.
Split into simpler sentences: Lángos is sold at many fast-food restaurants not only in Hungary but also in Austria. In Austria specially in Vienna lángos is very popular as fast food on fairs and in amusement parks like the Prater.