Lewandowski can make a good contribution. Yes, if you say he played well in Germany you can say "Spain is a different football", but Robert also played well in the Champions League and scored many goals, so for him it does not matter in which league and against which team to play, he can adapt in Barcelona
Lewandowski will surely play a great role if Barcelona sign him. Although, he is no longer young, but, he talented and can still score many goals for Barca. His performance in the Champion League last season was quite impressive, he scored 13 goals in the Champion League last season. While, I'm convinced that Lewandowski won't find it extremely difficult to adapt to Barcelona's style of playing. Nevertheless, it might take a little time for his to fully understand Barça pattern of football.