Are there any thriller novels written in simple English for non-native speakers?

Children’s or YA series are probably going to be best here.

Alex Rider is a spy thriller YA series. It starts with **Stormbreaker**

**Artemis Fowl** is also a very good fantasy/thriller series.

Finally **Sherlock Holmes** is a good mystery one. It’s an adult series but the stories are generally quite short so should be easier to understand the story than longer tales.