"German is not an ugly language."
Translation:La germana ne estas malbela lingvo.
The verb 'esti' / 'to be' is a copula; it doesn't have objects. In 'The girl eats an apple.' The verb 'to eat' has the object 'an apple' and 'the girl' is the subject. In 'The girl is intelligent.' There is no object, but an adjective (a feature) that is assigned to 'the girl' through 'is' (to be). [The girl doesn't DO anything in that sentence. She just IS something.]
The accusative case is only for objects so: 'La knabino vedas katon.' But 'La knabino estas bela.'