I would ask myself "Is it mango first, then beer, or is it Belgian Beer first, then mango" and enter it accordingly. I.E. - Mango first would be in "fruit beer" and Belgian first would be in "Belgian Specialty".
Or you could enter it in both
Brewing since 1989 - BJCP National Rank
Conditioning: Burton Ale, Belgian Quad, Old Ale
On tap: Cider, Impy Cherry Stout, Irish Red Ale, Irish Red Lager, Big Irish Red Ale, Rye Dunkel Bock, German Easter Lager, Mexican Lager, Belgian Dubbel Chocolate Stout
Newly Bottled: Dubbel, Belgian Golden Strong, Belgian Easter Ale