trendy
Following current fashion or a trend; modern and popular.
Das neue Café in der Innenstadt ist sehr trendy.
The new café downtown is very trendy.

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Following current fashion or a trend; modern and popular.
Das neue Café in der Innenstadt ist sehr trendy.
The new café downtown is very trendy.

Like most adjectives, 'trendy' can stand before a noun or be used with verbs such as 'sein' (to be) and 'wirken' (to seem/appear). When it stands before a noun, it is declined like a normal German adjective.
Sie trägt immer sehr trendige Kleidung.
She always wears very trendy clothes.
'Trendy' is an English loanword and is used mainly in colloquial language and in contexts such as fashion, lifestyle, and marketing. It sounds modern and is often used as a synonym for 'angesagt' (popular/trendy).
In diesem Stadtviertel gibt es viele trendige Bars und Geschäfte.
There are many trendy bars and shops in this neighborhood.
The adjective 'trendy' often describes places, products, or styles. Typical combinations are: eine trendige Bar (a trendy bar), ein trendiges Restaurant (a trendy restaurant), trendige Kleidung (trendy clothes), or ein trendiges Viertel (a trendy neighborhood).
Das Magazin berichtet über die trendigsten Reiseziele für dieses Jahr.
The magazine reports on the trendiest travel destinations for this year.
in
'in' and 'trendy' can both describe something that is currently fashionable. In this meaning, 'in' is very short and rather colloquial. You often hear it with clothing, music, or things that are currently popular. 'trendy' is an adjective and sounds a bit more international or advertising-like. It fits well when something seems deliberately modern. Important: 'in' is also very often a preposition for place, direction, or time. As a trend word, you can usually recognize it because it is used with 'sein', for example 'Das ist in.' (That's in / trendy.)

Das neue Café ist hell, teuer und ziemlich trendy eingerichtet.
The new café is bright, expensive, and decorated in a pretty trendy style.
Kurze Videos sind bei vielen Jugendlichen gerade in.
Short videos are really in among many young people right now.