English Translation
Most Japanese temples are made of wood.
i think it is ‘templen’
I don’t know the exact rules, but the sentence is best as written. I think it’s because de flesta is a quantifier, and you wouldn’t say *“många japanska templen”; you would say “många japanska tempel”. It’s another matter if you leave out flesta: “de japanska templen är gjorda av trä”. Your suggestion also works if you replace flesta with an adjective: “de vackraste japanska templen är gjorda av trä”.
Beware though: “De vackraste japanska tempel (som) jag har sett är gjorda av trä”. This is because of the relative clause that follows.