"Az új repülőgép lassú."

Translation: The new plane is slow.

[KadalHU]

"The new plane is slow." should be accepted.

[Krisbaudi]

Why slow and not slowly? Is IS not a Verb?

[RyagonIV]

Yes, but ... hmm, so many reasons.
You're describing a noun here, not the quality of its existence. Generally, forms of 'to be' do not carry adverbs around. Same in Hungarian, lassú is an adjective, the adverb would be lassan.
It takes a bit of getting used to when you're starting from German because in German adverbs have no suffix and adjectives always have one except when they're used as a sentence adjective: Das Flugzeug ist langsam. It's still an adjective, though.

[Krisbaudi]

Thank you