Dry air doesn't worry them. More likely wet with cold.

Where in Spain? They are fine all along the Gold coast,Maybe inland to Barcelona if covered in winter frost.

I hope you have it outside now acclimating to sun or part sun being how apartment balconies can get.

We live inland near from Barcelona. The mango is outside since May, when the night temperatures went up. But it was inside all winter. It just got the new set of leaves- big, bright and green But all the old ones are brown and dry. So you guys think it‘s Because of the cold? Even through the plant was inside wintertime? What should I do next winter? What should I do with the bad leaves?

Cut off the old leaves..let the plant put all it can into that nice new foliage. You should be able to grow them in ground in Barcelona easily..or a pot. Winter lows there are very mild. Just protect small plants like that if you get a frosty night. Like you did last in 1985..so,no,no big worry about the cold.