What a great question. I also speak Japanese and German, though poorly. What is your native language? If it's one of those two, I'd love similar recommendations.
Sounds like your best bet would be young adult books. They tend to have lighter themes but are a fairly quick read.
Here is the progression I would take, though it's very much light on "thriller" you will have a bit of a hard time trying to find thrillers with a simple vocabulary. If you make it through this stack though, you could go wherever you wanted.
- Tintin, an easy read. There are lots in this series and it's a good start to English lit. If you can follow a conversation, you ought to be able to read these.
- Terry Pratchett wrote a whole series of fun and easy reads.
- The Chronicles of Narnia is another easy read meant for children, but with enough story to keep an adult interested.
- Snow Crash, an easy read, though I'm not sure why. A lot of Stephenson's other stuff was much more difficult. This one read like an action movie. This would be the first book I consider written for adults.
- The Hitchhiker's Guide to the Galaxy. I'd be remiss not to recommend it, and if you can make it through the predecessors, you can definitely take this one on.
- Ender's Game reads easy but has a lot to it. Besides it's a classic.
- Brave New World is a relatively easy read but definitely leaving happy teenager world.
- Dune is not a children's book. It's a great read, and will test your growing vocabulary.
- The Sirans of Titan. By the time you've got this far you can really read whatever you want. I just think this is a great book to end the list on.
Afterwards you can hit up Tom Clancy, C.S. Friedman, Stephen King, or whatever you like. Enjoy!!!