• Technically correct but unidiomatic (so wrong). You should say 'Car sales decline in Europe'. Also, if you are translating these only to learn, I would advise that you translate the other way round (from English to Arabic).
    I would say, 'The continuing decline of car sales in Europe,' or (if one prefers an independent clause; inspired by إسكندراني), 'Car sales are continuing to decline in Europe.'