In English, as in German, there are various ways to describe a person who is intelligent, depending on whether you’re concerned with amount of knowledge, ingenuity, perception, or judgement.
The main word is also intelligent in German, but pronounced with a German accent, if you will. If you want to be more colorful in your description, you can try some more specific versions of the word intelligent.
Examples
klug — This word is quite universal and can indicate a sharp mind, logical ability to think, educated, experienced, rational, and sensible.
schlau — This word translates as “astute” or “smart” and is used for people who use their available means creatively to reach their goals. Synonyms are pfiffig or gewitzt.
A “smart alec” is Schlauberger (m.) or the pejorative Klugscheißer (m.) in German.
helle or aufgeweckt — These words (the former literally meaning “bright” and the latter “woken up”) are used to describe people (usually young people or children) who have a quick intellectual grasp of new concepts (for their age).
kenntnisreich or sachkundig — These words mean “knowledgeable” and “competent”.
scharfsinnig — This word means “perceptive” or “sharp”.
gerissen — This word has a negative connotation, literally means “ripped” and is used for people who know all the ropes so that they can’t be fooled by others, or who are clever in a way that serves their own interests.
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Related Words
Other words in this spectrum are weise (“wise”), gescheit (“brainy”/”intelligent”), genial (“brilliant”) and clever (“clever”).
Relatedly, the figurative English “whiz kid” is a Senkrechtstarter (m.) in German, which literally means “something that takes off vertically”, like a rocket.
A “genius” is Genie (n.), and a “prodigy”, of course, is Wunderkind (n.).